the 
 explosophore no2 group is an essential component 
 in many propellants and explosives of practical relevance . n , o  c and c  n covalent 
 bonds , have significant biological activity , and may contribute to 
 atmospheric pollution . furthermore , decomposition 
 processes of nitro compounds are especially important because they 
 occur at relatively low temperatures and are involved in the detonation 
 of explosives . most of the energetic molecules 
 have diffuse electronic spectra 
 and propensity to dissociate and rearrange . in the detonation initiation of these materials , in particular , as has been increasingly recognized in the field 
 of photochemical reactions , nonradiative decomposition 
 processes of energetic molecules through conical intersections play 
 an important and even dominant role . therefore , investigations 
 of nonadiabatic processes occurring in energetic molecules are especially 
 interesting . the study of gas phase energetic molecules can 
 unveil their properties 
 and reactions at the molecular level , and determine decomposition 
 features as a function of the electronic and vibrational excitations . however , the experimental measurements of decomposition 
 processes and their interpretation are very complex , a picture asking 
 for theoretical investigations . moreover , in spite of the importance 
 of excited states in energetic materials research , only few studies 
 have been carried out in this field . we recently 
 investigated the electronic excitation spectra of energetic 
 molecules containing nitro groups , namely , nitramide ( h2nno2),n , n - dimethylnitramine , ( ( ch3)2nno2 ) , 1,1-diamino-2,2-dinitroethylene 
 ( fox-7 or dadne ) , and hexahydro-1,3,5-trinitro-1,3,5-triazine 
 ( rdx ) . these molecules are building blocks 
 of energetic materials and bear easily broken n  no2 bonds . nitroethylene ( h2c = chno2 ) and 
 fox-7 , in contrast with the other studied molecules , have a c  no2 bond instead of a n  no2 . no2 bonds are also relatively weak and have its no2 group adjacent to a c = c bond . this molecule displays conjugation effects between the ethylenic  
 electrons and those on the nitro group that affect the electronic spectrum . molecules of this class are 
 potentially synthetic reagents , powerful electron acceptors due to 
 the presence of no2 groups , and can also behave as precursors 
 to useful polymers and copolymers for explosives technologies . in spite of the variety of aforementioned 
 interests , previous work 
 on discussed its 
 unimolecular decomposition using mp2 , mp4 and g2 methods and zhang and co - workers focused on ground - state 
 isomerization reactions and vertical calculations of the first five 
 electronic transitions , respectively at the dft / b3lyp/6 - 31 g * and td - dft - b3lyp/6 - 31 g * 
 levels of theory . shamov and khrapkovskii 
 considered the pyrolysis mechanism of nitroethylene using the b3lyp/6 - 31g(d ) 
 method . khrapkovskii and co - workers reviewed 
 quantum chemical data on gas - phase decomposition mechanisms of c - nitro 
 compounds including nitroethylene . concerning the excited electronic states of nitroethylene up to 
 the year 2006 , apparently only three other theoretical studies and 
 an older experimental work can be found in the literature . the spectrum 
 of nitroethylene in gas phase was measured in the 1960s by loos and 
 co - workers , who observed a very weak 
 band at 4.2 ev and an intense band centered at 6.12 ev with a shoulder 
 at 5.12 ev . pople 
 configuration interaction ( ppp - ci ) method to assign the intense band 
 and its shoulder to  * transitions . in another 
 work , ha employed scf - ci wave functions and a small gaussian basis 
 set to find also the character of the first band as a n- * transition 
 and of the intense band and its shoulder as two  * 
 transitions . in 2006 , zhang and co - workers examined vertical excitations computed at the 
 tddft - b3lyp/6 - 31 g * level once more confirming the  * 
 character of the intense band and its shoulder . recently , we 
 carried out a systematic examination of the electronic 
 spectrum of nitroethylene using a variety of methods : the approximate coupled - cluster singles and doubles wave 
 function with the resolution of the identity ( ri - cc2 ) , the time dependent 
 density functional theory with the camb3lyp functional ( tddft / camb3lyp ) 
 and the dft multireference configuration interaction ( dft / mrci ) . vertical 
 transition energies and optical oscillator strengths of up to 20 singlet 
 transitions were computed . additionally , semiclassical simulations 
 of ultraviolet ( uv ) spectra at the ri - cc2 and dft / mrci levels were 
 performed . the main features in the uv spectrum were assigned to a 
 weak n- * transition , and to two higher energetic ones as  * 
 bands . the transition energies 
 of the bands in the dft / mrci simulation agreed quite well with experiment , 
 although it overestimated the band intensities . the ri - cc2 calculation 
 produced intensities comparable to the experiment , but the bands were 
 blue - shifted by 0.5 ev . a strong  * band , not 
 previously measured , was found in the 89 ev range . the 
 goal of this work is to discuss the photochemical pathways 
 for the decay of nitroethylene to the ground state . in this discussion , 
 the photodynamics of ethylene and fluoroethylene will guide us , the 
 latter serving as an example for a polar biradicoloid . both cases 
 can be qualitatively explored by a 3  3 ci analytical model 
 as developed by bonai - kouteck and michl . this model predicts for nonsymmetric biradicaloids like fluoroethylene 
 the appearance of a degeneracy between the s0 and s1 states by simple torsion of the cc double bond , as was confirmed 
 by quantum chemical calculations . in 
 contrast , it is well documented that in ethylene the torsion is not 
 sufficient for reaching an intersection with the ground state but 
 that pyramidalization and hydrogen transfer modes play an important 
 role . from this 
 point of view , nitroethylene is an interesting example , since it polarizes 
 the c = c bond due to electronegativity differences of the groups 
 bonded to it , but it also includes contributions from  conjugation 
 and lone pair orbitals . for this reason , a multitude of excited states 
 already appears in the case of vertical excitations , and conical intersections should involve ch2 and no2 torsions . the calculation of excited - state energy 
 surfaces and the search for conical intersections of nitroethylene 
 is thus even more involved . in this work , we employ the multireference 
 configuration interaction 
 with singles and doubles ( mr - cisd ) method to calculate potential energy curves for selected coordinates , to 
 determine minima on the crossing seam ( mxs ) and to characterize nonadiabatic 
 interactions between different electronic states of nitroethylene . these features should allow us to develop an overall picture of the 
 nitroethylene photochemistry through possible nonradiative decay mechanisms 
 originating in a bright electronically excited molecule leading it 
 to reach the ground state . the calculations were carried out in two steps . the first one consisted 
 of a state - averaged multiconfiguration self - consistent field ( sa - mcscf ) 
 calculation followed by a multireference configuration interaction 
 with single and doubles excitations ( mr - cisd ) . the state averaging 
 was performed with equal weights and included six states ( sa-6 ) , namely , 
 the ground state s0 ( 1a ) , three a 
 and two a states in the case of vertical excitations . for 
 describing these excitations , a complete active space cas(8,6)including 
 eight electrons and six orbitals , the four highest occupied orbitals 
 ( two a and two a ) , and two virtual orbitals of a 
 symmetry , was chosen . the sa - mcscf character of the orbitals are 2a 
 [  ( no2 ) ] , 15a [ n ] , 16a [ n(o ) ] , 
 3a [  ( c = c+ no2 ) ] , 4a [  * ] 
 and 5a [  * ] ( see figure 1 ) . the 
 15a and 16a orbitals are respectively n+ and n type - orbitals because the former corresponds 
 to symmetric linear combinations of the oxygen lone pair orbital and 
 the latter , the highest occupied molecular orbital ( homo ) , represents 
 the antisymmetric combination . this sa-6-casscf 
 calculation was followed by a mr - cisd with the reference configurations 
 generated from the same cas ( 8,6 ) space and applying the interacting 
 space restriction . this orbital space 
 has been chosen to represent the reference space for the computation 
 of the five lowest singlet valence excited states as discussed in 
 our previous work . since the focus of 
 this work is laid on the exploration of the photodeactivation mechanism 
 of the valence states , rydberg states , which appear above those in 
 the upper range of the states chosen , were not included in our calculations . the carbon , oxygen , and nitrogen 1s orbitals were frozen 
 in the 
 ci calculations . the davidson corrections for size - extensivity 
 were also computed . the 6 - 31++g * * gaussian basis set was used . it will be used for the calculation of the 
 vertical spectra , potential curves , and mxs structures . the basis set was extended significantly 
 to 6 - 311+g(2d1f ) on the heavy atoms . for the hydrogen 
 atoms , a 6 - 31 g * * , the active space of the reference wave 
 function was increased while keeping the basis set at the 6 - 31++g * * 
 level . the acronym ras stands for a restricted active space and aux for auxiliary 
 space . the notation ras(2,1 ) means that two a and one a 
 orbital were moved from the doubly occupied space into the active 
 space . in the case of aux(2,1 ) , two a and one a virtual 
 orbitals were moved into the active space . this configuration space was used as a wave function in the 
 mcscf calculation and as a reference wave function in the case of 
 the mr - cisd calculation . the geometries were described by natural internal coordinates and optimized using the direct inversion in 
 the interactive subspace ( gdiis ) method . geometry optimizations and determination of the minima on the crossing 
 seam ( mxs ) , i.e. , the conical intersections respectively employed 
 analytic mr - cisd gradients and analytic mr - cisd 
 nonadiabatic coupling vectors . the exception was the s2/s1 and s5/s4 conical intersections , 
 which were converged only at the cas ( 8,6 ) level ; in these cases , 
 a single - point mr - cisd with the same type of wave function used for 
 the other geometries were done . the potential energy curves were obtained 
 with full optimization of all coordinates except the selected coordinate 
 for constructing the potential curve . the atomic orbital ( ao ) and the 
 derivative integrals were calculated using the dalton program . the dft / b3lyp//6 - 311g+(2df,2p ) 
 planar ground state geometry obtained in our previous work was employed for the vertical calculations . transition energies and oscillator strengths from the s0 ground state to the s1s5 valence excited 
 states were computed . the results are collected in table 1 together with our previous dft / mrci results and available experimental data . the measured weak band is assigned to the two n * 
 transitions involving states s1 and s2 . the 
 mr - cisd transition energies computed in this work for the s1 and s2 transitions are ( mr - cisd+q values between parentheses ) : 
 3.69 ev ( 4.03 ev ) and 4.14 ev ( 4.48 ev ) . these two dark transitions 
 are respectively excitations from the n ( 16a ) 
 and n+ ( 15a ) orbitals localized on the no2 moiety to the same  * ( 4a ) final orbital . there is 
 another dark transition ( s4 ) that also originates from 
 the n+ ( 15a ) orbital , but in this case is an excitation 
 to the higher  * ( 5a ) orbital . the s5 state corresponds to the measured band maximum 
 and the s3 corresponds to the band shoulder ; both transitions 
 are of  * type . the s3 state has mr - cisd 
 transition energies ( mr - cisd+q between parentheses ) of 6.33 ev ( 6.05 
 ev ) and oscillator strength f = 0.034 ( mr - cisd ) and 
 the s5 has 7.58 ev ( 6.00 ev ) and f = 0.375 . 
 the s3 state is a transition from a  ( 2a ) 
 orbital localized on the no2 group to a  * ( 4a ) 
 orbital while the s5 state , the brightest of the computed 
 set , is an excitation from the  ( 3a ) orbital to the 
  * ( 4a ) having contributions from electron density distributed 
 over the cc bond as well . as it is typical of energetic molecules , 
 the bright  * the transition energy 
 of the s5 bright  * 
 transition computed at the mr - cisd+q level agrees quite well with 
 the experimental band maximum . the weaker  * transition 
 ( s3 ) is too high at both the mr - cisd and mr - cisd + q levels . in contrast to our previous results , there is an inversion of the mr - cisd energy ordering of the n * 
 ( s4 ) and  * ( s5 ) transitions 
 in comparison with the dft / mrci calculations ( table 1 ) . however , the vertical energy ordering becomes compatible 
 with dft / mrci after inclusion of the davidson ( mr - cisd - q ) correction . an overall comparison of the 
 mcscf transition energies with all 
 the available mr - cisd values ( table 1 and table 2 ) shows relatively little changes except for the 
 mcscf(extref ) results of the s5 - 3a 
 state . this shows that our standard cas(8,6 ) reference space is in 
 principle flexible enough to represent the major features of all states 
 investigated except the one mentioned . in this case , the mcscf(extref ) 
 enhances the weight of  * excitations on the 
 cost of artificial n excitations ; 
 thus , the character of the s5 state is considerable improved 
 in spite of the higher mcscf excitation energy . the mr - cisd approach 
 actually increases in most cases the excited state energies relative 
 to the ground state ; the most prominent exception is the s5 state for which the mrci(extref ) not only enhances the weight of 
 the excitation from the  ( 3a ) to the  * ( 4a ) 
 orbital , but also stabilizes this state relative to the others . significant 
 changes are observed by applying the davidson correction which decreases 
 the excitation energies of the two  * states 
 2a and 3a considerably , especially 
 of the latter state . the energetic location of both states is especially 
 improved by extension of the active space to the extref case , but 
 the 2a state is still about 0.6 ev above the experimental 
 value . further extension of the active space beyond the present ras / cas / aux 
 scheme of 14 electrons in 12 active orbitals would be necessary in 
 order to increase especially the stability of the 2a 
 state , but such an approach will render the calculations very costly . 
 since our focus was not so much concentrated on the vertical excitations 
 but on the exploration of the photodynamical deactivation picture , 
 we did not follow this path further . overall , the mr - cisd and 
 mr - cisd+q methods provide a good starting 
 point for the following discussion of reaction paths on excited - state 
 energy surfaces and the determination of conical intersections . these 
 investigations will lead into regions of the energy surfaces far away 
 from the franck condon region , and it is this aspect where 
 the multireference method will be most useful . for this purpose , we 
 have chosen the standard mrci / cas(8,6 ) approach which gives a good 
 balance between computational efficiency and flexibility of the wave 
 function . following the experience 
 with the photodeactivation mechanisms found for ethylene and fluoroethylene , the mr - cisd potential energy curves of the states s0s5 were calculated for two rigid torsions : of the no2 group around the c n bond ( figure 2 ) and of the ch2 group around the c = c bond ( figure 3 ) ; the latter torsion also occurs in ethylene . the mr - cisd ch2 out - of - plane bending 
 curves ( figure 4 ) were also computed . in all 
 cases , the calculations started from the dft / b3lyp//6 - 311g+(2df,2p ) 
 ground state geometry . rigid mr - cisd potential 
 energy curves for no2 rotation 
 around the cn bond starting from the dft / b3lyp//6 - 311g+(2df,2p ) ground 
 state geometry . rigid mr - cisd potential 
 energy curves for ch2 rotation 
 around the cc bond starting from the dft / b3lyp//6 - 311g+(2df,2p ) ground 
 state geometry . mr - cisd ch2 out - of - plane bending starting at the dft / b3lyp//6 - 311g+(2df,2p ) 
 ground state geometry . we have seen above that the  * s3 state ( 2a ) is a vertical transition characterized 
 by molecular orbitals mostly localized in the no2 moiety . 
 upon no2 torsion , the s3 excited state is destabilized 
 by about 0.5 ev ( figure 2 ) . a partial exception 
 to this behavior is the s4 state that , after initially 
 stabilizing , destabilizes also following the behavior of the other 
 states . there is an avoided crossing between s3 and s4 at around 45 where the character of the two states 
 is exchanged . s4 now acquires  * character 
 and s3 becomes n+*. for greater 
 torsion angles , the s4 and s5 states become 
 almost degenerate . the small gap in the curves between the pair of 
 states s3s4 and s4s5 indicates the possible presence of conical intersections 
 and crossing seams involving torsion of the no2 moiety . for the ch2 torsion curves ( figure 3 ) , the picture has more features . the s3 state 
 in nitroethylene , a  * vertical excitation , is 
 stabilized by about 0.5 ev for increasing ch2 torsion angles . the s2 and s3 states reveal an avoided crossing 
 and exchange their character at an angle of about 56. from this 
 angle onward , the dominant character of s3 is n+ * while the s2 state becomes *. 
 both states become almost degenerate at 70 with an energy difference 
 of 0.3 ev , thus indicating a crossing . similar to ethylene , 
 the ground state s0 ( 1a ) in nitroethylene 
 destabilizes strongly upon ch2 torsion ( figure 3 ) by almost 3 ev . this situation 
 leads to a reduction of the gap to the first excited state ( s1 ) by about 2 ev , a 50% decrease . this contrasts with the very 
 small destabilization of the ground state upon no2 torsion 
 ( figure 2 ) . for a ch2 torsion angle 
 of 70 , the character of the s0 state ,  at 0 , now has a  * configuration 
 that contributes with a weight of about 20% . for ch2 torsion 
 angles of 90 , the s0 state is dominated by a  * 
 configuration with weight of 57% , while for s1 another 
  * configuration ( weight = 12% ) mixes with the 
 dominant n * character ( weight = 
 36% ) . this character is essentially maintained in the mxs ( s1/s0 ) found ( see table 3 ) . for ethylene , it has been shown that pyramidalization of the ch2 group ( combined with partial hydrogen migration ) is important 
 for reaching the conical intersection between a  * 
 state and the s0 . therefore , we also investigated the effect of the 
 ch2 out - of - plane bending ( i.e. , pyramidalization ) coordinate 
 through the potential energy curves of nitroethylene . the mr - cisd 
 ch2 out - of - plane bending curves of nitroethylene 
 starting from the ground state geometry are shown in figure 4 . overall , the six states including the ground state 
 steadily destabilize by about 1 ev . the s4 and s5 exchange n+ * and  * 
 character via an avoided crossing for a ch2 out - of - plane 
 angle of 55. the energies of the pair of s4/s5 curves slightly decreases with increasing ch2 out - of - plane 
 angle , and the energy splitting remains very small . thus , for nitroethylene 
 there is no significant effect of the ch2 out - of - plane 
 motion on the expected conical intersection structures . the sa-6-mcscf / mr - cisd optimized geometries 
 of the ground state and of the minima on the crossing seam ( mxs ) are 
 shown in figures 58 . the optimized ground state distances and angles computed 
 in this work at mr - cisd level are displayed in figure 5a . they agree very well with the ones obtained previously 
 by means of the dft / b3lyp approach . the main feature of the mxs ( s1/s0 ) ( figure 5b ) as compared with the 
 mr - cisd ground state is the position of the ch2 group , 
 twisted by 97 : it is now essentially perpendicular to the original 
 molecular plane . there is a cc bond elongation of 0.14  and 
 also an elongation of the no bonds : the larger no elongation is 0.14 
  , and the smaller one is 0.02  . although the torsional structure 
 of the mxs ( s1/s0 ) could be deduced from the 
 large ch2 torsion angle of the corresponding potential 
 curves , figure 3 , the just indicated additional 
 bond length variations are crucial to reach the conical intersection . 
 as already mentioned above , out - of - plane bending of the ch2 group is not important in nitroethylene . ( a ) mr - cisd ground state . 
 computed conical intersections ( mxs ) : ( b ) mr - cisd mxs ( s1/s0 ) , and ( c ) casscf mxs ( s2/s1 ) . the dihedral angles 
 are shown according to the numbering given in the inset at panel a. 
 insets of panels b and c are front views of the corresponding structures . the mxs ( s2/s1 ) ( figure 5c ) is overall similar to the mxs 
 ( s1/s0 ) ( figure 5b ) ; 
 the torsion of the ch2 group by about 90 is also 
 the prominent feature . there are 
 differences in the elongation of the no and cn bonds of the mxs ( s2/s1 ) as compared to the mxs ( s1/s0 ) as well as differences in the angular values despite the 
 two insets looking very similar . there is a torsion of the no2 group and of the c2h8 bond ( for atomic 
 numbering , see the inset of figure 5a ) in the 
 opposite sense leading to an almost perpendicular orientation of both 
 relative to the ground state molecular plane ( see inset of figure 6a ) . there is an opening of the ono bond and a visible 
 pyramidalization of the no2 group . the cn distance in the 
 mxs ( s3/s2 ) structure decreases by 0.03  
 as compared to the ground state . the three ch distances decrease slightly 
 by 0.04  , while the larger no distance increases by 0.16  
 and the smaller one increases by 0.02  . there are considerable variations of the valence angles , 
 the most pronounced one being the 9 increase in the no2 opening angle and the ch2 opening angle decreasing by 
 2.6. computed conical intersections ( mxs ) : ( a ) mr - cisd mxs ( s3/s2 ) , ( b ) mr - cisd mxs ( s4/s3 ) , and 
 ( c ) casscf mxs ( s5/s4 ) . the dihedral angles are shown according 
 to the numbering given in the inset in figure 5 , panel a. insets are front views of the corresponding structures . although the shape of the mxs 
 ( s4/s3 ) ( figure 6b ) is 
 overall similar to the mxs ( s1/so ) ( figure 5b ) and mxs ( s2/s1 ) ( figure 5c ) conical intersections , 
 there are important differences . in the following , the cc bond of the 
 mxs ( s4/s3 ) ( figure 6b ) elongates by 0.13  and the cn bond by 0.3  . the n3o5 bond stretches by 0.07  , while the other 
 no bond barely changes . in comparison with the other conical intersections , 
 the mxs ( s4/s3 ) stretches almost all bonds , 
 and the ch2 group twisted by about 80 as compared 
 to the ground state structure . the no2 group is also twisted 
 almost by 90 and displays some pyramidalization . in the 
 mxs ( s5/s4 ) ( figure 6c ) , the structure is almost planar ; the ch2 group 
 is twisted by a small angle as compared to the other conical intersections , 
 but overall the geometry is more planar in comparison with the other 
 mxs s . the no2 angle increases by 5.5 and 
 all the bonds elongate , excepting the cn and ch distances . s0 : 
 double excitation 
 from n to the two distinct virtual  * orbitals . s4 : double excitation 
 from n and  to the two distinct virtual 
  * orbitals . s5 : double excitation 
 from n to a *. table 3 collects the character 
 of the conical 
 intersections found . it can be seen that the character of the mxs s 
 could be qualitatively inferred from the computed potential energy 
 curves . in particular , the overall similarities of the mxs ( s1/s0 ) and mxs ( s2/s1 ) geometries 
 discussed above have correspondence with the respective character . the expected multiconfiguration character of the states in most intersections 
 is clear , and  * type transitions are relevant 
 in the five intersections . moreover , there is an important role in 
 the character of all conical intersections played by the lone pair 
 ( n ) orbitals localized on the no2 moiety . the g- and 
 h - vectors characterizing the conical intersection are shown 
 in figures 7 and 8 for 
 the five computed mxs structures . the g - vector is defined by the energy 
 gradient difference between the two states , and the h - vector is the 
 nonadiabatic coupling vector . a molecular deformation along these 
 vectors determines the branching space , lifts the degeneracy of the 
 states , and describes possible reaction coordinates . ( a ) 
 mr - cisd mxs ( s1/s0 ) , ( b ) casscf mxs ( s2/s1 ) , and ( c ) mr - cisd mxs ( s3/s2 ) . the arrows are not to scale between different pictures . for the mxs ( s1/s0 ) , the g - vector ( figure 7a ) the cc torsion leading to a reduction of the s1/s0 gap also occurs in ethylene and fluorethylene . 
 as discussed before for ethylene , the s1/s0 gap 
 is reached via ch2 pyramidalization and partial hydrogen 
 migration while in fluorethylene cc torsion 
 and stretching reduce the s1/s0 gap . therefore , in this case nitroethylene resembles 
 more the fluorethylene case because for both molecules , in contrast 
 to ethylene , the ch2 pyramidalization does not play a role . although the geometry of the mxs ( s2/s1 ) 
 is similar to the mxs ( s1/s0 ) , their g- and 
 h- vectors ( figure 7b ) display distinct features . 
 the most important ones are the bending deformation of the no2 group and the absence of ch2 motion . the mxs ( s3/s2 ) g- and h- vectors ( figure 7c ) display pronounced 
 stretching modes of the no2 bonds , combined with an angular 
 deformation of the no2 group and a stretching of the cc 
 bond . the h - vector also 
 indicates a ch2 torsion and an angular deformation of the 
 ch2 bond . the g- and h- vectors of the mxs ( s4/s3 ) ( figure 8a ) are dominated by an out - of - plane deformation of the no2 group with a contribution of a cc stretching . the mxs ( s5/s4 ) ( figure 8b ) g- and 
 h - vectors also display a deformation of the no2 group and 
 cc stretching , similarly to the other conical intersections . the existence of a series of conical intersections in nitroethylene 
 suggests nonradiative deactivation processes via internal conversion 
 ( ic ) . in figure 9 , an overview of possible 
 deactivation paths based on the energies and geometries of the conical 
 intersections is presented . it can be seen that the s5/s4 , s4/s3 and the s3/s2 conical intersections are quite close in energy . based on 
 this scheme and along with the computed potential energy curves , the 
 following nitroethylene deactivation photodynamic process can be deduced . computed 
 g- and h - vectors of the conical intersections found . ( a ) mr - cisd mxs ( s4/s3 ) and ( b ) casscf mxs ( s5/s4 ) . the main structural modifications of the conical intersections with 
 respect to the ground state geometry are displayed . bearing in mind that because nitroethylene is a 
 multistate problem , 
 the dynamics need not proceed necessarily through minimum - energy conical 
 intersections , but may take shortcuts through near - degeneracies of 
 higher energy ( e.g. , as observed in the ethylene dynamics ) , we suggest the following photodynamic mechanism . the torsion around the cc bond ( figure 3 ) does not lead to a pronounced stabilization as 
 is found in ethylene or fluoroethylene . therefore , this torsion alone does not lead 
 to a region of intersection with the s4 state . the no2 torsion ( figure 2 ) , however , points 
 to an s5/s4 intersection at a relatively small 
 torsion angle . the internal conversion to the s4 state 
 occurs mostly by cc stretching and no2 torsion ( mxs ( s5/s4 ) - see figure 6c and 
 figure 8b ) , with contribution of ch2 twisting . the cc bond lengthens from the ground state value of 1.33 
  to 1.47  and the cn bond shortens from 1.46 to1.37  . the energy of this mxs is stabilized by 0.4 ev as compared to the 
 vertical excitation . to continue to the s4/s3 conical intersection ( figure 6b ) requires 
 significant torsions around the ch2 and no2 groups 
 ( figure 8a ) . in the mxs ( s4/s3 ) structure the c2h8 bond is now almost 
 perpendicular to the planes of the ch2 and no2 groups , with the cc and cn bond lengths decreasing and increasing , 
 respectively , as compared to the mxs ( s5/s4 ) . the computed potential curves confirm this picture : upon no2 ( figure 2 ) and ch2 ( figure 3 ) torsion , the s4 state slightly stabilizes . the mxs ( s4/s3 ) is only 0.14 ev higher in energy 
 than the mxs ( s5/s4 ) and certainly accessible 
 energetically within the deactivation dynamics . thus , it can be also populated directly through 
 an excitation from the ground state even though with a smaller probability 
 as compared to the s5 state . the ch2 torsion 
 stabilizes the s3 state and leads to an intersection with 
 s2 . on the other hand , these features are reflected in the structure of the 
 mxs ( s3/s2 ) ( figure 6a ) where a rotation of the no2 group with respect to the 
 mxs ( s4/s3 ) is observed . the mxs ( s3/s2 ) undergoes a significant energetic stabilization by 
 0.8 ev as compared to the mxs ( s4/s3 ) . the energies of the s1 and s2 states upon 
 ch2 torsion are energetically close in the whole range 
 ( figure 3 ) , particularly at the 90 ch2 torsion angle , a torsional region that is also assumed by 
 the preceding mxs ( s3/s2 ) . in fact , the mxs 
 ( s2/s1 ) is characterized by a ch2 torsion of 90 ( figure 5c ) , 
 though the no2 bending ( see figure 7c ) , combined with a relevant shortening of the cc bond and elongation 
 of the cn and no bonds , led to the s1/s0 intersection 
 ( figure 7a ) . the mxs ( s1/s0 ) is characterized by a ch2 torsion of 90 
 similarly to the mxs ( s2/s1 ) . as was already 
 noted above , this intersection is 
 not only achieved by this torsion but also by non - negligible changes 
 of the cc , cn , and no bond distances with respect to the ground state 
 values suggestive of hot ground state dynamics after transition to 
 s0 . the energy of the mxs ( s1/s0 ) 
 is stabilized by 0.4 ev in relation to the mxs ( s2/s1 ) . in view of the 3  3 ci analytical model 
 as developed by bonai - kouteck 
 and michl , the torsion of the ch2 in nitroethylene from the planar geometry ( figure 3 ) reduces the s1/s0 gap as seen in the 
 corresponding potential curves . a no2 torsion has little 
 influence on the s1/s0 gap ( figure 2 ) . this reduction according to that theory is due 
 to an increased electronegativity difference of the cc bond upon ch2 torsion . however , this gap reduction in nitroethylene is 
 not as pronounced as , e.g. , in fluorethylene . in the latter , in contrast , in nitroethylene a rigid cc torsion leads to a gap 
 of 2 ev and in ethylene to 2.3 ev . however , as said 
 above , a combination of bond changes , namely , a significant lengthening 
 of the cc ( 0.15  ) and one no bond ( n3o5 = 0.14  ) , and also a shortening of the cn bond ( 0.16 
  ) , as compared to the ground state geometry , takes place and 
 is responsible for reaching the s1/s0 intersection . moreover , the energy gradient difference g - vector of the mxs s1/s0 ( figure 7a ) indicates 
 possible no2 release through activation of the cn bond 
 stretching . no pyramidalization of the ch2 or no2 groups is observed . in this sense , the situation in nitroethylene 
 can be considered similar to the polar fluoroethylene , with a significantly 
 larger contribution of changes in the bond distances as compared to 
 the ground state . after a nonadiabatic transition , the vibrational 
 energy acquired 
 in the process by the molecule concentrates in the small number of 
 vibrational modes that define the conical intersection . nitro compounds are usually thermally unstable 
 and composed of easily broken covalent bonds such as n release of the very labile no2 group in 
 these molecules is thus rather probable in the hot ground state dynamics . the g - vector in the mxs ( s0/s1 ) ( figure 7a ) , as discussed above , indicates the activation 
 of a cn stretching mode . therefore , it is expected that upon nonradiative 
 transitions these modes might favor the release of the no2 group . extensive 
 mr - cisd calculations were performed to study photodeactivation 
 mechanisms of nitroethylene involving the first five valence excited 
 states . these calculations presented good agreement with our previous calculations 
 and the experimental spectrum , excepting an inversion of the energy 
 ordering of the last two transitions , the n * ( s4 ) and  * ( s5 ) . the potential 
 curves for torsion and pyramidalization of the ch2 group 
 and the torsion of the no2 moieties were 
 computed . five conical intersections , one between every pair of adjacent 
 states , were found : s5/s4 , s4/s3 , s3/s2 , s2/s1 , and s1/s0 . the computed g- and h - vectors 
 describing the shape of the intersection 
 cone point to an important role played by the torsions of the ch2 and no2 groups , with contributions of other modes 
 such as cc and cn stretching . similarities between the behavior of 
 nitroethylene and previous work on the ethylene and fluorethyelene molecules 
 were explored . from the computed potential energies and the 
 conical intersections 
 found , a photochemical mechanism of nitroethylene deactivation could 
 be proposed starting in the bright s5 (  * ) 
 excited state . after a franck condon excitation to this state , 
 a nonradiative chain of deactivation processes of nitroethylene from 
 this state downward can proceed through conical intersections mostly 
 characterized by twisting the ch2 , with important contributions 
 from deformations of the no2 group and cc and cn bond stretches . furthermore , the modes of the mxs ( s1/s0 ) g - vector 
 indicate the possibility of no2 release in the ground state , 
 an important process in nitro molecules with explosive properties . the results of this work have shed light on the photochemistry 
 of nitroethylene , in particular on the importance and role of the 
 conical intersections between adjacent pair of states . moreover , these 
 results will allow further chemical dynamics investigations on this 
 molecule .
extended multireference configuration 
 interaction with singles 
 and doubles ( mr - cisd ) calculations of nitroethylene ( h2c = chno2 ) were carried out to investigate the photodynamical 
 deactivation paths to the ground state . 
 the ground ( s0 ) 
 and the first five valence excited electronic states ( s1s5 ) were investigated . in the first step , 
 vertical 
 excitations and potential energy curves for ch2 and no2 torsions and ch2 out - of - plane bending starting 
 from the ground state geometry were computed . 
 afterward , five conical 
 intersections , one between each pair of adjacent states , were located . 
 the vertical calculations mostly confirm the previous assignment of 
 experimental spectrum and theoretical results using lower - level calculations . 
 the conical intersections have as main features the torsion of the 
 ch2 moiety , different distortions of the no2 group and cc , cn , and no bond stretchings . in these conical intersections , 
 the no2 group plays an important role , also seen in excited 
 state investigations of other nitro molecules . based on the conical 
 intersections found , 
 a photochemical nonradiative deactivation process 
 after a  * excitation to the bright s5 state is proposed . 
 in particular , the possibility of no2 release in the ground state , an important property in nitro explosives , 
 was found to be possible .