nitric oxide ( no ) is 
 one of three important diatomic ligands ( no , 
 co , and o2 ) that interact with iron porphyrinates . although the ligands have many common attributes , 
 each ligand can form complexes that have distinct characteristics , 
 and many biological systems are able to distinguish among the three 
 diatomics . uniquely , no is the only one of the three that will react with iron 
 porphyrinates in both the iron(ii ) and the iron(iii ) states . the strong 
 interest in no as a ligand has led to many investigations and reviews . in addition , the important properties of no in 
 biology as a messenger and as an immune defense agent are especially 
 significant in the { feno } heme systems . the five- and six - coordinate { feno } complexes have been 
 studied in some detail ; their electronic 
 structures are reasonably well understood and will not be considered 
 further here . the iron(iii ) systems ( { feno } ) have 
 been less intensely 
 studied but also have substantial biological interest especially as 
 insect carriers of no and in denitrification 
 processes . experimentally , the relatively easy loss of no 
 from both solutions and solid samples of { feno } complexes , 
 even though the fe n(no ) bond is very short ( 1.641.65 
  ) , and easy reductive nitrosylation to { feno } species has made syntheses and characterization challenging . the no stretching frequency for five - coordinate 
 [ fe(oep)(no ) ] derivatives 
 ranges from 1830 to 1868 cm . early characterization 
 revealed unusual spectroscopic properties for six - coordinate species 
 of the type [ fe(porph)(l)(no ) ] , where l is a neutral nitrogen 
 donor such as imidazole or other nitrogen heterocycle . these species 
 had no stretching frequencies in range of 18901921 cm , thus bracketing the value of free no ( 1876 cm ) . the mssbauer 
 spectra of the five - coordinate species showed relatively low isomer 
 shifts of 0.20 mm / s , whereas 
 the six - coordinate species had unusually low values , 0 mm / s , a value in hemes that is usually associated 
 with the oxidation state of + 4 for iron . thus , a description of electronic structures of these seems challenging , 
 an issue to which we will subsequently return . we do note that there 
 are some substantial similarities between the reactions of o2 and no with hemes , in particular an unusual spin coupling of paramagnetic 
 reactants . in dioxygen systems , diamagnetic dioxygen complexes are 
 formed from high - spin five - coordinate iron(ii ) hemes and paramagnetic 
 dioxygen ; their description has been 
 contentious to this day . in the case 
 of the [ fe(porph)(r im)(no ) ] species , reaction 
 of a high - spin five - coordinate [ fe(porph)(r  im ) ] and paramagnetic no also forms diamagnetic complexes . the 
 reported vibrational spectra of six - coordinate [ fe(porph)(r  im)(no ) ] either as proteins or as small molecules have assigned both 
 the fe n  o bend should be readily distinguished 
 from the stretching mode by two ( near ) degenerate transitions for 
 the bend ; the degeneracy is lifted by the presence of the planar trans 
 ligand . definitive assignments should be obtained 
 by the application of oriented crystal nrvs spectra , which we have 
 recently pioneered and which provides detailed orientational , vibrational information . the reported crystallographic properties of five - coordinate { feno } species are appropriate for single - crystal nrvs measurements , 
 but those reported for six - coordinate species are not suitable . fortunately , we have isolated a new crystalline form of [ fe(oep)(2-mehim)(no ) ] with appropriate crystal symmetry characteristics to allow 
 single - crystal nrvs measurements . this new structure will be described . 
 with appropriate five- and six - coordinate species now available , we 
 have undertaken their oriented single - crystal nrvs that allows the 
 definitive assignment of both the stretching and the bend modes of 
 the axial feno group including the resolution of the near degenerate 
 bending modes in [ fe(oep)(2-mehim)(no ) ] . vibrational frequencies 
 can be used to illuminate coordination - site and ligand bonding subtleties 
 beyond that discernible by x - ray crystallography alone . we comment on the stretching and bending 
 patterns of the five- 
 and six - coordinate { feno } species as compared to the five- 
 and six - coordinate { feco } and { feno } species . im stretching 
 mode across the series of no , co , and o2 iron complexes . all reactions and manipulations 
 for the preparation of the iron(iii ) porphyrin derivatives were carried 
 out under argon using a double manifold vacuum line , schlenkware , 
 and cannula techniques . dichloromethane , toluene , and hexanes were 
 distilled under argon over cah2 and sodium / benzophenone , 
 respectively . chlorobenzene was purified by washing with concentrated 
 sulfuric acid , then with water until the aqueous layer was neutral , 
 dried with sodium sulfate , and distilled twice over p2o5 . nitric oxide ( mittler specialty 
 gases ) was purified by fractional distillation through a trap containing 
 4a molecular sieves bathed in a dry ice / ethanol slurry . insertion 
 of fe 
 into free - base porphyrins was conducted in purified chlorobenzene 
 according to a modified landergren method as follows . approximately 2 ml of 12 m hcl was used to dissolve 0.20 
 mmol 95% fe enriched fe2o3 contained 
 in a 250 ml schlenk flask . after 5 min of stirring , hcl was removed 
 under vacuum to near dryness , and subsequent steps of the metalation 
 reaction were done in an inert environment using standard schlenk 
 procedures . failure to maintain an inert environment will likely result 
 in an incomplete or inefficient metal insertion . one hundred milliliters 
 of freshly distilled chlorobenzene was added via cannula , and the 
 mixture was refluxed for 2 h in a hot oil bath . one - half of the chlorobenzene 
 was removed by distillation , and 10 ml of a chlorobenzene solution 
 containing 0.15 mmol of h2oep and 0.4 ml of 2,4,6-collidine 
 was added via cannula . after refluxing for 3 h , chlorobenzene was 
 removed under vacuum , and the residue was dissolved in 50 ml of ch2cl2 . the solution was washed twice with 50 ml of 
 di water , thrice with 50 ml of 1 m hcl , and once with 50 ml of di 
 water . after drying over na2so4 and filtration 
 through a sintered - glass filter , minimal ch2cl2 was used to redissolve the product 
 and transfer to an evaporation dish . after metalation as described 
 above , the resulting [ fe(oep)(cl ) ] was reacted with agclo4 to produce [ fe(oep)oclo3 ] . to 
 10 mg ( 0.014 mmol ) of [ fe(oep)oclo3 ] was added 
 2 ml ch2cl2 . no gas was bubbled into 
 the solution for several minutes . a dramatic color change from brownish - red 
 to purple / pink occurred . a 1:1 mixture of hexanes and ch2cl2 was used as the nonsolvent in the vapor diffusion 
 experiment under an no atmosphere to obtain x - ray - quality crystals 
 that were used in the single - crystal nrvs experiments . caution ! transition metal perchlorates should be handled with great 
 caution and be prepared in small quantities as metal perchlorates 
 are hazardous and may explode upon heating . procedures 
 similar to the reported method were used 
 for the single - crystal preparation . five - coordinate 
 [ fe(oep)(2-mehim)]clo4 was prepared by the 
 reaction of [ fe(oep)oclo3 ] with 2-mehim in 
 methylene chloride . methylene chloride 
 ( 0.5 ml ) was added to solid [ fe(oep)(2-mehim)]clo4 ( 12 mg ) in an 8  150 mm glass tube inside an extra 
 long schlenk tube under argon . the color of the solution changed from red / brown 
 to purple / pink . x - ray - quality crystals were obtained by layering no - saturated 
 hexanes over the solution in the tubes . these crystal were 
 used in both the x - ray structure determination and the single - crystal 
 nrvs experiments . a 
 black crystal with 
 the dimensions 0.52  0.43  0.17 mm , glued 
 to the tip of a glass fiber , was used for the structure determination 
 at 100 k. intensity data were collected on a bruker apex system with 
 graphite - monochromated mo k radiation (  = 0.71073  ) . the asymmetric unit contains 
 the full porphyrin cation , the perchlorate anion , and one - half of 
 a dichloromethane molecule . the structures were solved by direct methods 
 using shelxs-97 and refined against f using shelxl-97 ; subsequent difference fourier 
 syntheses led to the location of most of the remaining nonhydrogen 
 atoms . the perchlorate anion was found to be disordered over two distinct 
 orientations at the same crystallographic site with occupancies of 
 0.684 and 0.316 . the solvent molecule is disordered around an inversion 
 center of the unit cell . for the structure refinement , solid - state analysis of crystal packing distances 
 made use of the program mercury from 
 the cambridge crystallographic data center . complete crystallographic 
 details , atomic coordinates , anisotropic thermal parameters , and fixed 
 hydrogen atom coordinates are given in the supporting 
 information for the structure . copper wire , 45 cm in 
 length and 18 gauge , was affixed to the goniometer and bent into a 
 u - shape . a glass fiber , 58 mm in length , was then superglued 
 to project along the goniometer -axis . the connection of the 
 wire into the goniometer head was fortified with epoxy resin . a crystal the 
 wire was then carefully bent so that the crystal was approximately 
 on the -axis , and then stretched to the required height for 
 centering . crystals were then oriented for nrvs analysis along specified 
 in - plane axes by methods described in the supporting 
 information . spectra were measured 
 at sector 3-id of 
 the advanced photon source , argonne national laboratory , argonne , 
 il . single - crystal samples were previously mounted and aligned on 
 goniometer heads that were screwed into place on a rotating stage . the mounted crystals were then located into the x - ray beam by translations 
 of the stage and rotated to the predetermined -angle required 
 for the particular direction of analysis desired . a stream of cold 
 n2 gas from a commercial cryocooler controlled crystal 
 temperature during nrvs measurements . typical temperatures at the 
 crystal were 100120 k. vibrational spectra were measured using 
 an in - line high - resolution monochromator operating at 14.4125 kev 
 with 1.0 mev bandwidth scanning the energy of incident x - ray beam . spectra were recorded between 30 and 
 80 mev in steps of 0.25 mev , and all scans ( 36 replicates ) 
 were normalized to the intensity of the incident beam and added . nrvs 
 raw data were converted to the vibrational density of states ( vdos ) 
 using the program phoenix . nrvs measurements were 
 made in two orthogonal directions for [ fe(oep)(no)]clo4 and three orthogonal directions for [ fe(oep)(2-mehim)(no)]clo4 . the gaussian 09 program package was used to optimize the structures and for 
 frequency analysis , with the ( bp86 ) and 
 ( m06-l ) functionals . in general , we used 
 triple- valence basis sets with polarization functions ( tzvp ) on iron and 6 - 31 g * for all other atoms . the s = 0 complexes [ fe(oep)(no ) ] and [ fe(oep)(2-mehim)(no ) ] were fully optimized without any constraints using the spin 
 unrestricted dft method . the starting structure for each was obtained 
 from the crystal structure . frequency calculations were performed 
 on the fully optimized structures at the same level to obtain the 
 vibrational frequencies of the fe isotopomer . the frequency data have been created 
 using the high precision format vibrational frequency eigenvectors 
 to calculate mode composition factors ( e ) and vibrational density of states ( vdos ) as described below . the g09 output files from the dft calculations are 
 the fraction of total kinetic energy contributed by atom j ( here , fe , the nrvs active nucleus ) . the normal mode 
 calculations are obtained from the atomic displacement matrix together 
 with the equation:1where the sum over i runs 
 over all atoms of the molecule , mi is the atomic mass of atom i , and ri is the absolute length of 
 the cartesian displacement vector for atom i. the 
 polarized mode composition factors are defined in terms of two distinct 
 in - plane directions , which can be calculated from a projection of 
 the atomic displacement vectors x and y ( eq 1 ) . the out - of - plane 
 atomic displacement perpendicular to the resulting porphyrin plane 
 for a normal mode is obtained from a projection of the atomic displacement 
 vector z ( eq 2).2the x , y , and z components 
 of the iron normal mode energy 
 for [ fe(oep)(no ) ] and [ fe(oep)(2-mehim)(no ) ] with efe2  0.01 are given in tables s7 , s8 of 
 the supporting information . the predicted 
 mode composition factors ej2 can also be compared 
 to the integrated spectral areas obtained 
 from nrvs . therefore , vibrational densities of states ( vdos ) can be 
 simulated from the mode composition factors using the gaussian normal 
 distributions function , where the full width at half height ( fwhh ) 
 is defined appropriately by considering the spectral resolution in 
 the experiment . in this study , the x - ray 
 structure of the [ fe(oep)(2-mehim)(no ) ] cation 
 has been determined ; a thermal ellipsoid plot is shown in figure 1 . also shown in the figure complete final crystallographic information 
 is given in tables s1s6 of the supporting 
 information . thermal ellipsoid diagram of the [ fe(oep)(2-mehim)(no ) ] cation along with the perchlorate counteranion . figure 2 displays a formal diagram of the 
 displacement of each atom , in units of 0.01  , from the mean 
 plane of the 24-atom core for the [ fe(oep)(2-mehim)(no ) ] cation . the core conformation is moderately ruffled with the iron 
 atom effectively centered in the porphyrin plane . figure 2 also gives average bond distances and bond angles 
 for the core atoms . standard uncertainties , calculated on the assumption 
 that all values are drawn from the same population , are given in parentheses 
 following each averaged value . oriented single - crystal nrvs 
 spectra were obtained for the five - coordinate 
 [ fe(oep)(no ) ] and six - coordinate [ fe(oep)(2-mehim)(no ) ] derivatives . three distinct crystal orientations were measured 
 for [ fe(oep)(2-mehim)(no ) ] : either perpendicular to the 
 porphyrin plane or parallel to the porphyrin plane and either perpendicular 
 or parallel to the axial imidazole plane . figure 3 shows these experimental oriented crystal vibrational spectra . 
 in the high frequency region from 570610 cm , one strong vibration is seen in each of the three measured directions . 
 because the stretch and bend modes are expected to be the highest 
 frequencies for iron vibrations , the oriented crystal spectra show 
 the fe  no stretch as the out - of - plane mode , and the other two 
 in - plane modes , parallel and perpendicular to the imidazole plane these vibrational measurements show that in powder ( isotropic ) measurements 
 the three peaks will typically either be poorly resolved or not resolved 
 at all . thus , the unresolved powder peaks for the stretching and bending 
 modes seen by praneeth et al . and moeser 
 et al . formal diagram 
 illustrating the deviation of individual atoms ( in 
 units of 0.01  ) from the 24-atom mean plane of the porphyrin 
 core . positive values of the displacement are toward the no ligand . 
 the orientation of the trans 2-methylimidazole ligand 
 with respect to the porphyrin molecular frame is illustrated by the 
 solid line . the  o  gives the position of the 2-methyl 
 substituent of the imidazole . averaged values of the chemically unique 
 bond distances ( in  ) and angles ( in degrees ) are shown . the 
 numbers in parentheses are the esd s calculated on the assumption 
 that the averaged values were all drawn from the same population . two distinct orientations for five - coordinate [ fe(oep)(no ) ] were measured , one with the analyzing beam along the perpendicular 
 to the heme plane and a second with the beam in the porphyrin plane 
 ( which is orthogonal to the first ) . the results are shown in the discussion and provide all data required for the 
 assignment of the feno stretching and bending modes . both sets 
 of measurements provide direct evidence for making in - plane 
 versus out - of - plane assignments ; in particular , such measurements 
 are seen to be the most appropriate way to provide for the unambiguous 
 assignment of the feno stretching and bending modes . all previously known crystalline forms 
 of [ fe(porph)(r  im)(no ) ] derivatives are not appropriate 
 for a complete , detailed oriented single - crystal nrvs study . such 
 a study requires a crystalline form in which the porphyrin planes 
 are all parallel and all molecules have the same orientation ( molecules 
 related by inversion symmetry acceptable ) . fortunately , we have been 
 able to obtain a new crystalline form of [ fe(oep)(2-mehim)(no)]clo4 that meets these criteria and does allow measurements in 
 three distinct directions defined in the coordinate frame of the molecule . table 1 provides structural data for 
 related { feno } and feco complexes , where 
 the trans ligand ( when present ) is an imidazole species . comparison 
 of [ fe(oep)(2-mehim)(no)]clo4 with these data shows the 
 expected close agreement : an effectively linear feno group with a 
 short fe n(no ) bond ( 1.641(2 )  ) , and a trans fe  nim bond distance of 2.028(2 )  . an interesting feature 
 is the fe  nim bond distances to the sterically hindered 
 2-mehim . as seen in table 1 , the fe  nim hindered bond distances range from 2.028 to 2.053  , 
 lengths comparable to the fe  nim distances to unhindered 
 imidazoles in co complexes and much shorter than those to hindered 
 imidazole co species . thus , both axial bonds in the six - coordinate 
 { feno } species are very strong : not only is there no structural 
 trans effect , but the fe  no bond is unaffected by the change 
 in coordination number . all species listed in table 1 have coordination group distances appropriate for a low - spin 
 state for iron . displacement 
 of iron atom from the 
 24-atom mean plane , a positive value is toward the x  o ( x = 
 n , c ) ligand side . dihedral angle between imidazole 
 plane and the plane of closest np  fe  nax . the 
 oriented crystal spectra for both the 
 five- and the six - coordinate { feno } derivatives yield 
 clear assignments of the feno stretching ( out - of - plane ) and bending 
 modes ( in - plane ) ; compare figures 3 and 4 ( middle panel ) . these data show how the change 
 in coordination number leads to frequency shifts for these two modes 
 and allows comparison to other known heme systems with diatomic ligands . 
 for the { feno } derivative , [ fe(oep)(no ) ] , the experimental nrvs value for the fe  no stretch is 595 cm , and the experimental value for the fe this pattern , where the stretch 
 is observed at higher frequency than the bend , follows that observed 
 for [ fe(oep)(co ) ] ( 573 and 505 cm ) and the { feno } derivatives [ fe(oep)(no ) ] ( 517 and 394 cm ) and [ fe(dpix)(no ) ] ( 528 and 399 cm ) . the frequency 
 patterns for the six - coordinate analogs of co s and { feno } no s are reversed with the bend now found at higher frequency than the 
 stretch . thus , the observed bend is 582 cm and 
 the stretch is 499/513 cm for [ fe(oep)(1-meim)(co ) ] , and the bend is 559 cm and 
 the stretch is 433 cm for [ fe(tpfpp)(1-meim)(no ) ] . again , in these cases , the value of the stretching 
 mode decreases whereas the value of the bending mode increases . however , 
 for [ fe(oep)(2-mehim)(no ) ] , the observed fe  no 
 stretch remains high at 600 cm , whereas the two 
 near - degenerate bending modes increase to the observed values of 574 
 and 580 cm . thus , with the very similar frequencies 
 for both the stretching and the bending modes , the pattern in six - coordinate 
 { feno } species is quite distinct from other six - coordinate 
 diatomics . the close similarity of the fe  no stretch in the 
 two coordination numbers undoubtedly reflects that the axial fe the top panel displays the predicted 
 vibrational density 
 of states from an m06-l calculation for [ fe(oep)(no ) ] . the middle panel shows the experimental vdos for the in - plane and 
 out - of - plane measurements for [ fe(oep)(no ) ] . the bottom 
 panel gives the same experimental results for [ fe(oep)(no ) ] ; figure 
 data were abstracted from refs ( 42 ) and ( 68 ) . the bottom two panels of figure 4 compare 
 the nrvs spectra of [ fe(oep)(no ) ] and [ fe(oep)(no ) ] . the fe  no stretch in the cation is found 
 to be 78 cm higher , at 595 cm as compared to 517 cm , consistent with the 0.1 
  decrease in the fe  no bond distances . the purely in - plane bending mode of iron at 402 cm in [ fe(oep)(no ) ] contrasts with the in - plane and out - of - plane 
 components of the bend in [ fe(oep)(no ) ] along with a small decrease 
 in the observed frequency . we have also carried out a series 
 of dft calculations to aid in 
 understanding the complete vibrational spectra . for [ fe(oep)(2-mehim)(no ) ] , three sets of dft calculations based on the use of functionals 
 m06-l , bp86 , and b3lyp were completed . all predictions gave resolved 
 bending and stretching modes with the stretching mode always at higher 
 frequency ; the best match of the predicted spectra to the total observed 
 spectra was given by the m06-l calculations , and those will be used 
 here . consistent with the experimental 
 spectra , there are three main spectral regions to be considered . the 
 peaks in the 570610 cm region 
 are associated with the axial no ligand . as has been commonly observed , 
 the vibrational prediction for the axial stretch is at higher frequency 
 than that observed . in the lower 
 frequency regions , there are intense peaks in two distinct regions . 
 those between 300370 cm are mostly 
 associated with in - plane iron motions , whereas those in the region 
 of 200280 cm are dominated by 
 out - of - plane iron motion . there is some in - plane / out - of - plane overlap 
 in the 240280 cm range . dft calculations for [ fe(oep)(no ) ] were carried out 
 with the m06-l and bp86 functionals . the predicted bending mode frequencies are found 
 substantially below the stretch as observed in the single - crystal 
 data . these two bending modes ( at 414.3 and 414.4 cm ) are shown in figure s8 ( supporting information ) . this is in distinct contrast to the in - plane modes of [ fe(oep)(2-mehim)(no ) ] where the planar axial imidazole removes the x and y degeneracy . thus , the two major orthogonal 
 bending modes predicted at 585 and 
 591 cm are localized perpendicular or parallel 
 to the imidazole plane , respectively ( figure 5 ) . these correspond to the observed peaks at 574 and 580 cm ; the relative directionality and frequency orderings are preserved . as can be seen , the motions of the imidazole are also in 
 the direction of the bend , thus ring rotation for the 591 cm mode and motion perpendicular to the ring plane in the 585 cm mode . two minor in - plane peaks are also predicted 
 ( figure 5 ) ; these may explain the shoulders 
 seen at higher frequencies in both the in - plane x and the in - plane y spectra ( figure 3 ) . it is to be noted however that the in - plane x shoulder is more intense than the dft predictions would suggest . complete 
 m06-l predicted spectrum of [ fe(oep)(2-mehim)(no ) ] further 
 showing ( as e values , 
 bars ) the directional components in the three orthogonal directions . dft ( m06-l ) predicted fe  no bending modes 
 of [ fe(oep)(2-mehim)(no ) ] at 585 and 591 cm . for both species , these modes are generally porphyrin 
 vibrations , especially 49 , 50 , 
 and 53 . both the five- and the six - coordinate complexes 
 exhibit four relatively intense vibrations in the 338360 cm region . the two species do differ in the predicted motion 
 of iron . for [ fe(oep)(2-mehim)(no ) ] , both near - degenerate 
 pairs of vibrations have the iron motion approximately parallel or 
 perpendicular to the axial imidazole plane . we also note that the 
 highest frequency vibration is observed and predicted to have more 
 intensity than the others . [ fe(oep)(no ) ] , on the 
 other hand , has the two pairs of iron motions in this region , one 
 set along fe  np bonds and a second set halfway between 
 fe  np bonds . these are illustrated in the molekel 
 depictions found in the supporting information ( figure s9 ) . [ fe(oep)(no ) ] also has a second group of 
 in - plane vibrations centered around 295 cm . the predicted x and y components 
 are essentially equivalent , consistent with little or no anisotropy 
 in the porphyrin plane . the differences in the in - plane iron motion 
 in the two complexes probably reflect differences in the out - of - plane 
 displacements of iron that lead to differing amounts of mixing with 
 the ligand vibrations . molekel depictions of the four highest frequency in - plane 
 modes 
 of [ fe(oep)(2-mehim)(no ) ] . at lower frequencies , the in - plane iron vibrations of [ fe(oep)(2-mehim)(no ) ] are distributed over a large range of modes mixed with ligand 
 vibrations and not well resolved in the experimental spectra . for [ fe(oep)(no ) ] , on the other hand , the in - plane modes are 
 less mixed and are predicted to occur over a smaller number of frequencies 
 that are resolved both experimentally ( peaks at 285 and 304 cm ) and theoretically . the 
 highest frequency peak in the z direction 
 spectra of [ fe(oep)(2-mehim)(no ) ] is observed at 600 cm , which is clearly higher than the bending mode frequencies 
 already described . the prediction of a peak 33 cm higher than observed is not unexpected as the axial ligand frequencies 
 have always been the most difficult prediction for the dft calculations . the character of the m06-l prediction for [ fe(oep)(2-mehim)(no ) ] is shown in figure 8 . thus , the fe  no 
 stretch is always observed and predicted at higher frequency than 
 the bend . we believe that the stretch / bend assignments made for [ fe(tpp)(1-meim)(no ) ] should 
 be emended to have the stretch at higher frequency than the bend . although we believe that the assignments by 
 lehnert et al . should have the stretch at higher frequency than the 
 bends , their conclusions will be similar for either choice of assignment . the analogous fe  no stretch 
 in [ fe(oep)(no ) ] is seen at 595 cm , whereas the predicted position at 694 cm shows 
 a much large deviation . we have noted that the axial stretch prediction 
 discrepancies are frequently larger in the five - coordinate systems . dft ( m06-l ) 
 predicted fe  no stretch mode ( 633 cm ) and 
 fe  im stretch mode ( 225 cm ) of [ fe(oep)(2-mehim)(no ) ] . other axial iron vibrations are 
 those of the doming and inverse 
 doming modes . for the five - coordinate species , the inverse doming 
 mode is predicted at 242 cm and the doming mode 
 at 139 cm . values for the six - coordinate species 
 are 144 cm for the doming mode , and the observed 
 value is 152 cm . modes involving reverse doming 
 in the six - coordinate species are strongly mixed with other modes , 
 but the predicted character of the frequencies at 221 and 274 cm does show this character . probably the most 
 interesting axial vibration is that of iron to 
 the proximal imidazole . in five - coordinate iron(ii ) derivatives , the 
 fe im stretch is readily observed by resonance raman spectroscopy 
 and has been used extensively for protein and small - molecule characterization . a range of frequencies for histidine - ligated proteins is observed 
 ( 200256 cm ) that are correlated with the strength of the fe factors that are known to influence the bond strength include whether 
 or not the histidine n is hydrogen bonded . unfortunately , 
 for six - coordinate species , bands associated with 
 the fe  im stretch are typically not observed in resonance raman 
 experiments , so that very little information has been available . nuclear 
 resonance vibrational spectroscopy provides experimental information 
 on all iron vibrations and thus provides direct quantitative information . 
 however , the appearance of more than one predicted mode with simultaneous 
 iron and imidazole motion in this system as well as in previously 
 studied six - coordinate o2 , co , and { feno } hemes 
 leads to the difficult question of which frequency provides the most 
 accurate experimental description of the fe we examine the predicted character 
 of each calculated frequency and choose the one that best resembles 
 the classical fe classical  
 fe  im vibration for [ fe(oep)(2-mehim)(no ) ] is illustrated 
 in figure 8 ( 225 cm ) . the 
 similarity of this mode to others previously published can be seen 
 in the simulations displayed in figure 9 of ref ( 69 ) for [ fe(tpp)(1-meim)(co ) ] 
 and figure s9 of ref ( 100 ) for [ fe(tpivpp)(1-etim)(o2 ) ] . we then match the predicted 
 and experimental frequencies to choose the best experimental value 
 for the stretch for all available compounds . im stretch frequencies are given in 
 table 2 that also gives the experimental value 
 of the trans fe  n(im ) bond distance . the correlation 
 between the two is clearly evident and extends over a large range 
 of distances and frequencies . the strong correlation displayed may 
 strike some as self - evident , but we note that this is the first time 
 that such experimental data have been marshalled for a possible correlation . 
 the correlation clearly can be extended for the characterization of 
 systems in which only the vibrational data or the structural data 
 are available . im stretching frequencies 
 listed in table 2 are based on oriented single - crystal 
 nrvs measurements . the frequency ( in 
 cm ) with the motion that most closely resembles 
 that of the classical 
 fe correlation between fe  nim bond distances and 
 fe  im stretch frequencies ( fe  im ) 
 of six - coordinate [ fe(porph)(rim)(xo ) ] ( x = n , c , o ) complexes . , 
 [ fe(oep)(2-mehim)(no ) ] ;  , [ fe(tpp)(1-meim)(co ) ] ; 
  , [ fe(tpivpp)(2-mehim)(o2 ) ] and [ fe(tpivpp)(1-meim)(o2 ) ] ;  , [ fe(tpp)(1-meim)(no ) ] , tri-[fe(tpfpp)(1-meim)(no ) ] , and mono-[fe(tpfpp)(1-meim)(no ) ] . in our view , the description of 
 the electronic state of the six - coordinate 
 { feno } species remains enigmatic . the isoelectronic character 
 of { feno } and { feco } species has often been 
 expressed , with the implied expectation of strong similarities ; however , 
 our dft calculations for [ fe(oep)(2-mehim)(no ) ] and [ fe(oep)(2-mehim)(co ) ] 
 show large differences in the relative energies of equivalent mo s . the ir and nrvs vibrational data for [ fe(porph)(r  im)(no ) ] species are consistent with calculations that predict a formal 
 oxidation state description as fe(ii)(no ) . the observed 
 mssbauer spectra with the very small values for the isomer 
 shift are highly unusual for an iron(ii ) state ; rather they are similar 
 to those observed for iron(iv ) . yet calculations of neese suggest 
 that this is a bonding effect that results from the extremely strong 
 -bonding of an no ligand . the observed very strong fe n(no ) bond , yet the facile 
 loss 
 of no in [ fe(porph)(r  im)(no ) ] species , has puzzled 
 many investigators . recent calculations of lehnert and co - workers 
 have attempted to explain this perhaps paradoxical behavior . praneeth 
 et al.s calculations predict 
 the ground - state structure of six - coordinate [ fe(porph)(r  im)(no ) ] species as fe(ii)(no ) , with its very short axial 
 fe they further posit the existence of 
 high- and low - spin fe(iii)no(radical ) states that are low - lying 
 states lying within a few kcal / mol of the ground state . the first 
 excited state could be within 1.01.3 kcal / mol of the ground 
 state . they propose that it is these 
 states , with weaker fe  n(no ) and n  o bond strengths , 
 that lead to the facile loss of no . although this suggestion 
 has some reasonable features , we expect 
 that the existence of such close - lying states would lead to substantial t - dependent mssbauer quadrupole splitting values 
 as has been seen in the dioxygen complexes of hemoglobin and myoglobin . if the separation of ground and excited states was 1.3 kcal / mol , 
 the excited state would be populated by 15% , sufficient to 
 have temperature effects on the quadrupole splitting value . although 
 the experimentally available mssbauer data are limited , the expected temperature dependence of the quadrupole 
 splitting is not observed , thus casting some doubt on the suggestion 
 of close lying states . however , a significantly higher energy difference 
 has been given based on casscf or td - dft calculations . oriented single - crystal nrvs studies for [ fe(oep)(no ) ] and [ fe(oep)(2-mehim)(no ) ] provide unequivocal assignments 
 for the fe n  o stretching and bending modes for both . 
 the differing pattern of stretch and bend as a function of coordination 
 number for the { feno } derivatives is noted and compared 
 to the patterns previously observed for the related co and { feno } hemes . the use of nrvs methods allows the observation and 
 assignments of the several out - of - plane frequencies that are typically 
 not observed by other spectroscopic methods . im stretching frequency and 
 structural data for seven distinct o2 , no , and co species 
 establishes a correlation between the fe
oriented 
 single - crystal nuclear resonance vibrational spectroscopy 
 ( nrvs ) has been used to obtain all iron vibrations in two { feno}6 porphyrinate complexes , five - coordinate [ fe(oep)(no)]clo4 and six - coordinate [ fe(oep)(2-mehim)(no)]clo4 . 
 a new crystal structure was required for measurements of [ fe(oep)(2-mehim)(no)]clo4 , and the new structure is reported herein . 
 single crystals 
 of both complexes were oriented to be either parallel or perpendicular 
 to the porphyrin plane and/or axial imidazole ligand plane . 
 thus , 
 the feno bending and stretching modes can now be unambiguously assigned ; 
 the pattern of shifts in frequency as a function of coordination number 
 can also be determined . 
 the pattern is quite distinct from those found 
 for co or { feno}7 heme species . 
 this is the result of unchanging 
 fe  nno bonding interactions in the { feno}6 species , in distinct contrast to the other diatomic ligand species . 
 dft calculations were also used to obtain detailed predictions of 
 vibrational modes . 
 predictions were consistent with the intensity 
 and character found in the experimental spectra . 
 the nrvs data allow 
 the assignment and observation of the challenging to obtain fe 
 im 
 stretch in six - coordinate heme derivatives . 
 nrvs data for this and 
 related six - coordinate hemes with the diatomic ligands co , no , and 
 o2 reveal a strong correlation between the fe 
 im 
 stretch and fe  nim bond distance that is detailed 
 for the first time .