a major impetus for 
 the development of microelectronics is the 
 constant quest for ever smaller integrated circuits defined by moore s 
 law . a useful bottom - up approach to semiconductor 
 devices could be the synthesis of small silicon clusters , molecular 
 entities of defined structures that resemble parts of crystalline 
 or amorphous silicon . silicon clusters of diverse structural complexity 
 with intriguing electronic and bonding properties have been synthesized 
 by reductive oligomerization of polyhalosilanes or disproportionation reactions of si2cl6 . a different synthetic approach to silicon cluster 
 structures is the rearrangement of linear poly- or oligosilanes catalyzed 
 by lewis acids . pioneering work of kumada s 
 and west s groups showed that by using alcl3 as 
 the catalyst , linear oligosilanes can be transformed to structures 
 of higher complexity such as branched or cyclic systems . recently , we have been able to document the activity of carbocations , 
 such as the trityl cation , in these catalytic rearrangement reactions . furthermore , the use of a trityl cation paired 
 with a weakly coordinating anion in stoichiometric amounts allowed 
 the detection of oligosilanylsilyl cations under carefully controlled 
 reaction conditions . the most prominent example for 
 this type of lewis acid catalyzed sila - wagner meerwein rearrangement 
 is the synthesis of persilaadamantane 1 from a bicyclic 
 precursor that was reported recently by two of us ( scheme 1 ) . during this transformation , the complexity of the bicyclic compound 
 increases significantly by increasing the number of tetrasila - substituted 
 silicon atoms . while the formation of the persilaadamantane 
 is certainly a multiple - step reaction , the key for controlling this 
 type of rearrangement is a profound understanding of the generation 
 of branched oligosilanes from linear starting materials , which increases 
 the number of tetrasila - substituted silicon atoms . scheme 2 provides an example for this 
 type of reaction in which the linear oligosilane 2 is 
 transformed initially into its branched isomers 3 and 4 . we have studied the reaction 
 shown in scheme 2 by 
 ionizing close derivatives of oligosilane 2 at low temperature 
 and characterized important cationic intermediates , which has allowed 
 the formulation of a detailed mechanism for this reaction . in addition , 
 the accompanying computational studies have provided clear indications 
 that favorable attractive london dispersion forces in these intermediates drive these types of skeletal rearrangement 
 reactions in the direction of branched products such as 4 . the starting point of our experimental investigation was the ionization 
 of 2,5-dihydridohexasilane 5(33 ) as a mimic for oligosilane 2 . the use of the dihydrido 
 compound 5 in our study instead of the dimethyl compound 2 has two advantages : ( i ) the position for the cation formation 
 is clearly defined by the position of the accessible si h linkage serves as an autoscavenge 
 reagent , stabilizing the incipient cation by formation of a si the increased stability of the so - formed bis - silylhydronium 
 ion suggests that its nmr detection and characterization are possible 
 without affecting the topology of the originally formed silyl cation . 
 using this methodology , we are able to identify otherwise only transient 
 cationic species in rearrangement reactions of oligosilanes . from the reaction of dihydrido - oligosilane 5 with 
 trityl borate , [ ph3c][b(c6f5)4 ] , we thus expected the formation of bis - silylhydronium ion 7 as the self - trapping product of the logically first - formed 
 cation 6 ( scheme 3 ) . the reaction was conducted in chlorobenzene at t = 30 c and produced almost selectively a 
 polysilanyl cation with a highly symmetric structure . the formed cation 
 is characterized by only three si nmr resonances in the si{h } nmr spectrum at si 
 = 108.9 , 6.4 , and 120.9 ( scheme 3 and figure 1 ) . in the proton - coupled si inept nmr 
 spectrum , the high - field signal at si = 120.9 
 appears in a spectral region that is typical for tetrasila - substituted 
 silicon atoms and is only broadened by 
 unresolved couplings to distant hydrogen atoms . the resonance at si = 6.4 is a multiplet in the si inept 
 spectrum . the low - field signal at si = 108.9 
 reveals a doublet of septets pattern with coupling constants of j(sih ) = 43 hz and j(sih ) = 5 hz . this chemical shift and the small size of the j(sih ) coupling constant , this signal correlates 
 with a broad singlet in the h nmr spectrum at h = 2.15 . the analysis of si satellites for this 
 signal also reveals a j(sih ) coupling 
 of 43 hz . interestingly , the low - field si resonance also 
 shows a correlation to a singlet in the h nmr spectrum , 
 which we assigned to a sime2 group based on the relative 
 intensity in the h nmr spectrum . supported by additional 
 one- and two - dimensional nmr studies , we assigned the structure of 
 bis - silylhydronium ion 8 to the produced cation ( see figure 2 , scheme 3 , and supporting information ) . in particular , the analysis of the j(sisi ) coupling pattern was highly instructive , 
 which established the connectivity of the silicon atoms and secured 
 our structural assignment ( see figures 1 and 2 ) . cation 8 was transformed into the branched dihydrido compound 9 by reaction with na[et3bh ] at 30 c ( scheme 3 ) . compound 9 was identified by its si nmr chemical shifts 
 ( si = 8.8 ( sime3 ) , 30.2 
 ( sihme2 ) , and 132.9 ( sisi4 ) ) and by 
 comparison with literature data . si{h } nmr spectrum ( 99 mhz , c6d5cl , t = 30 c ) of the 
 reaction mixture after hydride abstraction from oligosilane 5 . insets are taken from the si inept ( top trace ) 
 and the si{h } nmr ( bottom trace ) spectrum 
 ( * unidentified compound ) . structure elucidation of cation 8 based on the analysis 
 of detected j couplings ( left ) and h / si correlations based on hmqc and hmbc spectra . finally , our structural assignment 
 for the formed cation was corroborated 
 by the good agreement between the experimental si nmr 
 parameters and those computed by quantum mechanical calculations ( see table 1 ) . the largest deviation was found for the tetrasila - substituted silicon 
 atoms ( calculated si = 124.8 vs experimental 
 si = 120.9 ) . noteworthy is the fine accordance 
 between calculated and experimental data for the central me2si  h  sime2 unit ( calculated si = 105.9 , j(sih ) = 40 hz vs 
 experimental si = 108.9 , j(sih ) = 43 hz ) . in addition , the results of calculations for cation 7 deviate significantly from those found in the experiment , 
 suggesting that cation 7 is not present in the reaction 
 mixture . it showed , however , the pronounced tendency 
 of the formed polysilanyl cations to favor structural arrangements 
 with tetrasila - substituted silicon atoms . for the assignments , see schemes 3 and 4 . calculated at giao / m06-l/6 - 311g(2d , p)//m06 - 2x/6 - 311+g(d , p ) . calculated at b3lyp / igloiii ( si , c , h ) , 
 6 - 311+g(d , p ) ( ge ) . next , 
 we turned our attention to the actual reaction mechanism 
 for the formation of cation 8 . substitution of the trisila - substituted 
 silicon atoms by germanium atoms in hexasilane 5 is not 
 expected to change the reactivity significantly , but it allows following 
 the reaction course ( scheme 4 ) . based on the experience with oligosilane 5 , we expected that hydride transfer from the 2,4-digermahexasilane 10 should result either in formation of bis - silylhydronium 
 ion 11 or in the generation of bis - germylhydronium ion 12 . the first cation is produced by complete skeletal rearrangement 
 with disruption of the central si in contrast , in cation 12 , the central backbone 
 of 
 germasilane 10 is conserved and the reorientation of 
 the silyl and methyl groups must have taken place by successive 1,2-shifts . 
 the reaction of digermahexasilane 10 with trityl cation 
 results in the formation a thermally unstable cation . even at reaction 
 temperatures as low as 40 c , we noted severe decomposition 
 of the formed product ( see supporting information ) . only the use of dichloromethane as solvent and reaction temperatures 
 as low as 95 c allowed the clean and selective formation 
 of a new cation , which was identified by si nmr spectroscopy 
 at 80 c . only two si nmr signals 
 at si 
 = 117.6 and si = 0.9 suggested a highly symmetric 
 structure for the produced cation . decisive for the structural assignment 
 was the low - field signal with a si nmr chemical shift , 
 which is characteristic for a cationic si si unit 
 ( si = 117.6 ) and which is a doublet in the si inept spectrum with a si  h coupling constant of j(sih ) = 45 hz . in addition , this si resonance shows cross - peaks to h nmr signals at h = 2.66 and h = 1.02 in the h / si hmbc nmr spectrum 
 ( figure 3 ) . the broad h nmr signal shows characteristic si satellites 
 of j(sih ) = 45 hz . the good agreement 
 between computed nmr chemical shifts and experimental data further 
 substantiated the assignment of the signals ( see table 1 ) . si{h } nmr spectrum ( 99 mhz , cd2cl2 , t = 80 c ) of the reaction 
 mixture after hydride abstraction from digermaoligosilane 10 . insets are taken from the si inept spectrum and the h / si hmbc nmr spectrum . based on these experimental results , we suggest the following 
 mechanism 
 for the formation of the characterized cation 8 and likewise 
 for its digerma analogue 11 ( scheme 5 ) . the logical starting compound for the 
 reaction sequence is cation anti-6 , 
 which undergoes a 1,2-silyl shift with disruption of the central si  si 
 bond to give the open cation 13 in its syn - conformation . the subsequent formation of the hydrogen - bridged cation 14 allows for a facile 1,4-hydride shift to give the open 
 cation syn-15 . a second 1,2-silyl shift 
 followed by rotation around the central si si bond in anti-16 results in the formation of the cyclic 
 bis - silylated hydronium ion 8 . the results of quantum mechanical computations provided ( i ) important insights with respect to the thermodynamic 
 driving force for the formation of the branched cations 8 and 11 and ( ii ) the basis for a deeper mechanistic 
 discussion of the reaction sequence shown in scheme 5 . in the following discussion , we will concentrate 
 on the rearrangements in the oligosilanyl system . the results for 
 the digerma analogue are quite similar with respect to the relative 
 energies of the isomers and their calculated structures and are given 
 in the supporting information . the close 
 similarities between the persila- and the digermasila systems indicate 
 that the perturbation induced by the replacement of silicon atoms 
 by germanium atoms is not likely to change our conclusions . as expected , 
 the results of density functional theory ( dft ) calculations at the 
 m06 - 2x/6 - 311+g(d , p ) level for the gas - phase indicate a significant 
 stabilization of all investigated polysilanyl cations by 5462 
 kj mol due to the formation of si  h  si 
 linkages ( 6/7 , 54 kj mol ; 13/14 , 62 kj mol ; 16/8 , 58 kj mol ) . the 
 most stable hydrogen - bridged cation , the branched cation 8 , is more stable by 16 kj mol than the 
 intermediate cation 14 and more stable by 30 kj mol than cation 7 , and it is the most stable 
 cation that we located on the potential energy surface ( pes ) ( see figure 4 , blue ) . we found 
 this result rather surprising because the quantitative evaluation 
 of substituent effects on the stability of cations having si  h  si 
 units indicated that substitution with four trimethylsilyl groups 
 at the two silicon atoms ( -substitution , as in cation 7 ) stabilizes this type of cation by 37 kj mol compared to tetramethyl substitution , as detailed in the supporting information . si ( as in cation 8) is less 
 favorable and stabilizes only by 24 kj mol . consequently , 
 comparison of the different substituent effects of the trimethylsilyl 
 groups in cations 7 and 8 suggests that 
 cation 7 is more stable than the branched cation 8 . this fact raises the question about the driving force for 
 the selective formation of cation 8 in the experiment . furthermore , the accumulation of four trimethylsilyl groups at the 
 central si  si unit in cation 8 seemed to us to 
 be a rather unfavorable situation for steric reasons . on the contrary , 
 attractive dispersion forces between the vicinal trimethylsilyl groups 
 in cation 8 could stabilize this cation compared to its 
 isomers 7 and 14 . the decisive role of attractive dispersion forces 
 between large , polarizable silyl groups for the formation of tetrylene 
 dimers was demonstrated recently by our groups and by nagase and power . in 
 order to quantify the relative stabilization of cations 7 , 8 , and 14 by attractive dispersion forces , 
 we optimized their molecular structures at the b3lyp/6 - 311+g(d , p ) 
 level . these results were compared with those obtained 
 using the same functional but including grimme s d3 function , 
 which adds dispersion corrections to the original b3lyp functional . interestingly , the b3lyp calculations predict cation 14 , which is putatively the sterically least congested one , to be the 
 most stable compound in this series ( see figure 4 , red ) . the outcome of the b3lyp / d3 calculations 
 alters the relative order of stability between cation 14 and the branched cation 8 ( see figure 4 , black ) . in addition , we note that the relative 
 stability order for the three cations is the same at b3lyp / d3 as at 
 the m06 - 2x level ( see figure 4 , blue ) . both methods predict the branched cation 8 to be the most stable one , which is in qualitative agreement with 
 the exclusive nmr detection of cation 8 in the experiment . the fact that only dft methods , which explicitly take dispersion energy 
 contributions into account , are able to confirm the experimentally 
 determined relative stability order hints at the importance of dispersion 
 energy contributions in this system . the comparison between the three 
 isomeric structures suggests that london dispersion forces are maximized 
 in cation 8 and overrule unfavorable substituent effects 
 ( compared to cation 7 ) and disadvantageous steric repulsion 
 ( relative to cation 14 ) . as a consequence , the attractive 
 dispersion forces between the vicinal trimethylsilyl substituents 
 in the branched cation 8 there is a clear 
 relation also to the rearrangement reaction shown in scheme 2 . in the framework of our model 
 chemistry , which replaces selected methyl groups with hydrogen atoms , 
 cations 7 , 8 , and 14 are placeholders 
 for the linear starting material 2 and the branched oligosilanes 3 and 4 . therefore , the determined relative stability 
 order in the cations 7 , 8 , and 14 clearly reflects the observed rearrangement chemistry shown in scheme 2 . this fact suggests 
 that dispersion energy contributions strongly influence the relative 
 stability order of the intermediate cations of this rearrangement 
 and are at least important components to its thermodynamic driving 
 force . nevertheless , the selective and exclusive formation of cation 8 with no indications for the presence of cation 14 contrasts the experimental finding for the actual rearrangement 
 reaction ( scheme 2 ) , 
 with both oligosilanes 3 and 4 formed initially 
 in equal amounts . in an attempt to tackle 
 this aspect , we started a detailed computational investigation of 
 the pes connecting the isomeric cations 7 , 8 , and 14 . the result of these calculations revealed 
 a delicate balance between inter- and intramolecular stabilization 
 of the silyl cation center . as shown above , the formation of si  h  si 
 hydrogen bridges stabilizes the cations 7 , 8 , and 14 significantly . for these cations , direct coordination 
 of the positively charged silicon atom to solvent molecules is not 
 of chemical significance even in solution . the calculated dissociation 
 energies e for the complexes 
 of the three hydrogen - bridged cations with chlorobenzene ( phcl ) are 
 small ( e = 39 kj mol ( 8) , 43 kj mol ( 14a , coordination via the sime2 group ) and 47 kj mol ( 14b , coordination via the si(sime3)2 group ) , and 44 kj mol ( 7 ) ) . in all three cases , the 
 main structural features of the hydrogen - bridged cations are conserved 
 also in the phcl complex . this is shown , for example , by the close 
 agreement between the calculated nmr parameter for the gas - phase structure 
 of cation 8 and for the complex 8(phcl ) 
 ( see table 1 and supporting information for a structural comparison ) . in addition , entropy effects favor the dissociation of these aggregates 
 into cation and solvent molecule , and their gibbs energy of dissociation , 
 g(298 ) , is calculated to be 
 negative ( g(298 ) = 14 
 kj mol ( 8) , 9 and 10 
 kj mol ( 14 ) , and 11 kj mol ( 7 ) ) , which negates their chemical significance . relative energies of isomers 7 , 8 , and 14 , calculated using different density functionals . b3lyp / d3 
 ( black ) correct the b3lyp values for dispersion interaction using 
 grimme s d3 function . the situation is different , however , 
 for their open isomers with 
 tricoordinated positively charged silicon atoms . structure optimizations 
 for cations such as anti-6 resulted 
 either in rearrangements ( e.g. , to cation syn-13 ) or in cations which show clear structural indications 
 for stabilization by remote methyl or silyl groups ( see supporting information for details and examples ) . the interaction energies for the latter stabilization modes are significantly 
 smaller than that computed for the formation of si  h  si 
 bridges . although these calculated structures are important for gas - phase 
 investigations , in solution , intermolecular interaction with solvent 
 molecules represses the intramolecular stabilization . as a consequence , 
 for these non - hydrogen - bridged cations , explicit coordination to solvent 
 molecules has to be taken into account . therefore , we determined the 
 relative energy of the intermediates for the proposed reaction pathway 
 shown in scheme 5 in 
 the form of their phcl complexes and used the polarized continuum 
 model to include the general effects 
 of solvation ( figure 5 ) . approximate reaction coordinate for the formation of the hydrogen - bridged 
 cation 8 starting from the complex between cation 6 and phcl . the relative energies e ( black ) and the relative gibbs energies g(298 ) at 298 k ( blue ) of the aggregates between cations and solvent 
 are computed using the m06 - 2x/6 - 311+g(d , p ) functional and are given 
 relative to the energy of the complex 6(phcl ) . the influence 
 of the solvent was modeled by scrf calculations using chlorobenzene 
 as solvent . the barriers shown are drawn arbitrarily ; see the supporting information for details . for cation 14 , complex 14a(phcl ) ( coordination via the sime2 group ) is higher in energy by 5 kj mol . in an extended computational search 
 for transition states connecting 
 these cationic intermediates , we faced , on the one hand , a very flat 
 potential energy surface on which no transition state relevant to 
 the investigated chemical transformations could be located . all barriers 
 found belonged to rotations of methyl or silyl groups . on the other 
 hand , on a pes on which topology is dominated by weak rotation modes , 
 no other large barriers are to be expected . this allows an approximate 
 discussion of the pes based on the different energies of the intermediates 
 of this reaction sequence . the computed reaction coordinate shown 
 in figure 5 reveals 
 an energy difference of about 3040 kj mol between the more stable hydrogen - bridged cations 7 , 8 , and 14 and their open isomers 6 , 13 , 15 , and 16 ( all in the 
 form of their phcl complexes ) . the least stable isomer in this series 
 is 6(phcl ) , which is higher in energy by 60 kj mol compared to the final product , the hydrogen - bridged 
 cation 8 . this relative small energy difference suggests 
 that even at temperatures as low as 30 c ( the temperature 
 of the experiment ) the thermodynamic equilibrium between the hydrogen - bridged 
 cations via their open isomers is fully established and explains the 
 selective formation of the most stable cation , the branched hydrogen - bridged 
 cation 8 . in order to put the results for our model system 
 into relation to the preparative important rearrangement chemistry 
 of scheme 2 , it is 
 interesting to note that the energy difference between cation solvent 
 complexes 14(phcl ) and 8(phcl ) is relatively 
 small ( e = 11 kj mol in 
 favor of cation 8) . thermal and entropy effects favor 
 the dissociation of both phcl complexes , and the gibbs energy difference 
 at 298 k , g(298 ) , between the cation 14 and 8 decreases to 6 kj mol . against the background that cation 14 and 8 are placeholders for the oligosilanes 3 and 4 , it is reasonable to assume that at higher 
 temperatures or different solvents a mixture of products is obtained . we investigated the 
 lewis acid induced rearrangement reaction of 
 linear oligosilanes to give new structures of higher complexity . we 
 used dihydrogen - substituted starting silanes for our stoichiometric 
 model reactions , which allow the targeted formation of cationic intermediates 
 by cleavage of one si h bond and their stabilization by formation 
 of si  h  si bonds with 
 the second si  h bond . the cationic products of the rearrangement 
 reaction were characterized at low temperatures by multinuclear nmr 
 spectroscopy . si bond of the oligosilane backbone as the key 
 step for the production of an oligosilane structure with a higher 
 number of tetrasila - substituted silicon atoms . according to accompanying 
 dft calculations , a major contributors to the thermodynamic driving 
 force for this reaction are attractive dispersion forces between the 
 polarizable trimethylsilyl groups , which are maximized in the detected 
 products . the course of wagner  meerwein rearrangements in carbon 
 chemistry is usually discussed in terms of carbocation stabilities 
 that are defined by substituent effects , such as resonance and hyperconjugation . 
 due to the increased size of the silicon atoms , these effects the results 
 of our combined experimental / computational investigation indicate 
 instead that attractive dispersion forces between large polarizable 
 silyl substituents play a dominant role in skeletal rearrangement 
 reactions of oligosilanes and determine the configuration of the products .
the installation 
 of structural complex oligosilanes from linear 
 starting materials by lewis acid induced skeletal rearrangement reactions 
 was studied under stable ion conditions . 
 the produced cations were 
 fully characterized by multinuclear nmr spectroscopy at low temperature , 
 and the reaction course was studied by substitution experiments . 
 the 
 results of density functional theory calculations indicate the decisive 
 role of attractive dispersion forces between neighboring trimethylsilyl 
 groups for product formation in these rearrangement reactions . 
 these 
 attractive dispersion interactions control the course of wagner 
 meerwein 
 rearrangements in oligosilanes , in contrast to the classical rearrangement 
 in hydrocarbon systems , which are dominated by electronic substituent 
 effects such as resonance and hyperconjugation .