Ab initio calculations on H3X2SiLi and H3X2Si− (X = N, P)

THEO CHEM ELSEVIER

Journal of Molecular Structure (Theochem) 313 (1994) 55 72

Ab initio calculations on H3X2SiLi and H3X2Si- (X = N, Gerhard Raabe Institut ffir Organische Chemie, Rheinisch-Westfdlische Technische Hochschule Aachen, Prof -Pirlet-Strasse 1, D-52056 Aachen, Germany

Received 19 October 1993; accepted 17 February 1994

Abstract While the local minima obtained for H3SiN2Li are planar or almost planar molecules, those for H3SiP2Li deviate significantly from planarity. In both cases the isomers lowest in energy correspond to cyclic non-classical structures with the lithium atom coordinated by either two nitrogen or two phosphorus atoms. The relative energy of the two most stable H3SiN2Li isomers is maintained when the lithium atom is complexed by an oxygen-contaning solvent modelled by water. The first solvation energy of about - 1 9 kcal mo1-1 for both species was found to be quite similar to those obtained by other authors for several first-row lithium compounds. The energetic difference between the two most stable H3SiN2Li isomers might, however, be reduced or even change its sign when a second solvent molecule is added. Upper bounds to the change of energy associated with the dimerization of the two most stable nitrogen compounds were calculated, and at - 4 4 to - 5 4 kcal mo1-1 they are less negative than the value obtained for the dimerization of lithiated aminonitrile. Among the nitrogen compounds the isomers containing divalently bonded silicon are lower in energy than those to which they are formally related by a 1,2 hydrogen shift. This result is in keeping with the well-known fact that aminosilylene is lower in energy than silanimine. Stabilization of a silylene by interaction of the silicon atom with a lone pair of an adjacent atom is less effective for phosphorus than for nitrogen. As in the case of silanephosphimine and phosphinosilylene the silylenes are now higher in energy than those isomers to which they are formally related by a 1,2 hydrogen migration. The interaction of the molecular subunits in some H3SiX2Li and H3SiX ~ isomers (X = N, P) was evaluated by means of bond separation reactions and the results were compared with the corresponding values for H3CX2Li and H3CX 2.

1. Introduction Recently, U n d e r i n e r a n d W e s t [1] r e p o r t e d the g e n e r a t i o n a n d c h a r a c t e r i z a t i o n o f A in solution. A loses the c o m p l e x i n g ether molecules u p o n heating in v a c u o at 160°C resulting in B, while r e a c t i o n with 15-crown-5 yields the ether-free l i t h i u m salt v~ Dedicated to Professor Dr. Carl Krfiger on the occasion of his 60th birthday. Presented in part at the 28th Symp. on Theoretical Chemistry, 24-30 September 1992, Brixen (Bressanone), South Tirol, Italy.

o f s i l a a m i d i n e a n i o n C (see Scheme 1). In o r d e r to o b t a i n n o t only k n o w l e d g e o f the m a i n g e o m e t r i c a l features o f these species b u t also a deeper u n d e r s t a n d i n g o f their electronic structures, we p e r f o r m e d q u a n t u m chemical a b initio calculations on m o d e l c o m p o u n d s . F o r this p u r p o s e we chose the u n s u b s t i t u t e d p a r e n t molecules H3N2SiLi a n d H 3 N 2 S i - . In some calculations the ether molecules were m o d e l l e d by water. Since for these p a r e n t c o m p o u n d s s t r u c t u r a l alternatives to those described

0166-1280/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved S S D I 0166-1280(94)03739-8

G. Raabe/J. Mol. Struct. (Theochem) 313 (1994) 55-72

56

R N ./" ", % Ph--Si÷ Li-OEt,~ N"

I R

2. C o m p u t a t i o n a l

R I

I

N,

-Et20 _ "

Ph--Si-

A

+Li N

-

I R

B

-

~ 15-crown-5 R I

./-

N

Ph--Si-

tJ÷115-crown-51

c

R=2,4,6-tBu3C6H 2 Ph=C6H5 Et= C2H5 tBu=C(CH3)3

R

R I

P,

"

PI "

\%

_D

(solid state)

R

in ref. 1 are conceivable, we did not confine our study to model compounds for A, B and C but included a total number of six lithium-containing molecules (1-6) and four anions (A1 A4) (see Figs. 1 and 5 respectively), all containing the N - S i N fragment. The synthesis, solid-state structure, and reactivity of the lithium salt D (see Scheme 1) of a 1,3-diphospha-2-silaallyl anion were reported in 1989 [2]. This compound is a highly substituted derivative of H3PzSiLi and its lithium atom is complexed by two ether molecules. To evaluate the structural differences between these compounds and their aza analogues mentioned above, we performed additional calculations not only for several isomers of sum formula H3P2SiLi but also for the anions H3P2Si-.

methods

Calculations were performed on the IBM 3090 and the SNI-$600/20 facilities of the Rechenzentrum der Rheinisch-Westf/ilischen Technischen Hochschule Aachen and on a local VAX 3100 workstation. While the GAMESS program package [3] was used for calculations on the IBM 3090 and the VAX WS 3100, the GAUSSIANg0 set of quantum chemical routines [4] was employed tbr computations on the SNI-600. Starting from reasonable model geometries most structures were pre-optimized with the 3-21G basis set [5]. The accuracy was successively increased (3-21G (*) [6], 6-31G* [7]), and since this study predominantly deals with lithium-containing molecules and anions, the final optimization was carried out at the 6 - 3 1 + G * level [8]. At the HF level all stationary points except those obtained for dimers and solvated species were characterized as either local minima or saddle points by calculation of their normal vibrations. To obtain more reliable relative energies, correlation effects were included by means of Moller-Plesset perturbation theory [9] to the second order at the 6-31 + G* optimized structures (MP2/6-31 + G*//6-31 + G*). Finally, zero point energies were calculated from the unscaled frequencies and added to the MP2corrected total energies (ZPE + MP2/6-31 + G*// 6-31 + G * ) . In some cases, additional optimizations were carried out at the MP2/6-31 + G * level. These calculations were performed using a basis set for silicon published by Francl et al. [7(c)]. This basis set differs slightly from that given previously by Gordon [7(b)], which was used in all other calculations. In both the single point calculations and the optimizations, the MP2 corrections include inner-shell contributions. If not mentioned otherwise, the energies discussed in this paper are of MP2/6-31 +G*//6-31 + G * quality, while structural parameters, vibrational frequencies and harmonic zero point energies were calculated at the HF/6-31 +G*//6-31 + G * level. Molecular drawings were generated using SCHAKAL [10].

Complete lists of all structural parameters and vibrational frequencies are available from the author upon request.

G. Raabe/J. Mol. Struct. (Theochem) 313 (1994) 55 72 3.1.1. Relative energies and geometries

3. Results and discussion

A t the h i g h e s t level o f a c c u r a c y e m p l o y e d in this s t u d y , 2 has the l o w e s t t o t a l e n e r g y a m o n g all s t u d i e d i s o m e r s o f g e n e r a l f o r m u l a H3N2SiLi. A t 1 9 0 . 2 p m , the S i - N 2 b o n d in 2 is s i g n i f i c a n t l y l o n g e r t h a n the s i l i c o n - n i t r o g e n b o n d in silylam i n e ( 1 7 3 . 3 p m ) . T h e l e n g t h o f the S i - N 1 b o n d is 1 6 6 . 1 p m a n d t h e r e f o r e close to the a r i t h m e t i c m e a n o f a s i l i c o n - n i t r o g e n single a n d d o u b l e b o n d

3.1. H3N2SiLi (1-6) T h e o p t i m i z e d s t r u c t u r e s are s h o w n in Fig. 1, while t o t a l a n d relative energies are g i v e n in T a b l e s 1 a n d 2. ( T o t a l energies a n d zero p o i n t e n e r g i e s o f a u x i l i a r y c o m p o u n d s are listed in T a b l e 13.)

1

57

( )H1 2

Si

H2

H3

H2

N1

~ L i

Li

H3

6

Si

N2

N2

Li H1 H2

Li 4

Si

5

Si

H3

L ~

H2

Fig. 1. Stationary points of sum formula H3SiN2Li. 1-3, 5 and 6 are local minima, while 4 corresponds to a saddle point. The parameters in parentheses are MP2/6-31 +G*-optimized data. Bond lengths in pm, bond angles in degrees. 1: r(N1-Li) = 196.8(195.1), r(N1-Si) = 162.5(164.9), LN1-Si-N2 = 109.0(107.3), LN1 Li N2 = 84.5(85.9), LLi-N1-Si = 83.3(83.4). 2: r(Nl-Li) = 184.5(183.9), r(N1 Si) = 166.1(168.9), r(N2-Li) = 197.6(195.2), r(N2-Si) = 190.2(191.0), LN1 Li N2 = 86.2(87.6), LN1-Si-N2 = 94.1(93.5). 3: r(N1-Li) = 180.0, r(N1-Si) = 166.4, r(N2-Si) = 158.6, LLi-N1 Si = 120.7, LN1-Si-N2 = 139.0. 4: r(N1 Si) = 160.6, r(N2-Si) = 190.3, r(N2-Li) = 191.1, LN1-Si-N2 = 106.3, LSi-N2-Li = 81.4. 5 : r ( N 1 Si) = 173.2(174.1), r(N2 Si) = 167.3(169.4), r(N2-Li) = 180.4(180.5), r(Si-Li) = 261.8(252.7), LN1-Si-N2 = 111.0(111.3). 6:r(N1 Si) = 170.8(171.9), r(N2-Si) = 156.5(159.9), r(N2 Li) = 172.5(172.4), LN1-Si-N2 = 129.3(129.3), LSi N2 Li = 177.5(177.5).

58

G. Raabe/J, Mol. Struct. (Theochem) 313 (1994) 55 72

Table 1 Total energies of 1-6 (in hartrees)

6-31 +G*//6-31 +G" MP2/6-31 +G*//6-31 +G* ZPE +MP2/6-31 + G*//6-31 +G* MP2/6-31 +G*//MP2/6-31 +G*

1

2

3

-407.055117 -407.511079 -407.468995 -407.515336

-407.066042 -407.022060 -407.007357 -407.051286 -407.522431 -407.476875 -407.466575 -407.503278 -407.476335 -407.435696 407.458564 -407.525459 -407.507188

(157.8pm in the parent silanimine H2Si=NH). Similarly, the frequency of the asymmetric stretch of the N - S i - N segment (1029.5cm -1) lies halfway between that of the stretching vibration of the S i = N bond in H 2 S i = N H and the Si N bond in silylamine, Formally, 2 is a cycloadduct of LiNH2 and HNSi and the formation of its formal

4

5

6

-407.017945 -407.479545 -407.437088 -407.484087

1, which corresponds to B with the aromatic substituents replaced by hydrogen atoms, is 7.1 kcal tool -1 higher in energy than 2. This energy difference is less than half of that between H z S i = N H and H - S i NH2, where the aminosilylene is about 18kcalmol 1 lower in energy than the silanimine [11]. The structure of 1 might be understood in terms of a superposition of the two equivalent limiting structures:

5- -- 5+

H--N=Si

_

H

H

/N\

l N

6*Li--N.8[.

I

H - - Si

H --Si

\/

O

Li

\\ND

N

I H

constituents is exothermic by 59.0kcalmo1-1. The H O M O of 2 is a a M O with pronounced s character which is predominantly localized at the silicon atom. Therefore, 2 might be envisaged as a cyclic silylene in which the nitrogen lone pair at N1 and the empty 3p orbital at silicon form a weak 7r bond. The N - S i N angle in 2 (94.1 °) is almost identical with the H - S i N angle of 95.4 ° in aminosilylene ( H - S i - N H 2 ) , while the S i - N bond length in this c o m p o u n d is 5.4 pm longer than the Si N I bond in 2.

Similar to other cyclic lithium compounds in which the lithium atom is involved in two L i - N bonds, for example dimers of lithiated ketenimines [12], the L i - N bonds in 1 are significantly longer than in lithium amide (175.4pm). The length of the S i - N bonds is 162.5 pm. Thus, these bonds are not only about 6.2% shorter than a typical S i - N single bond but also shorter than the Si N1 bond in 2. They are, however, still about 5 p m longer than the S i = N bond in silanimine.

Table 2 Energies of H3SiN2Li relative to the most stable isomer (in kcalmo1-1)

6-31 + G*//6-31 + G* MP2/6-31 + G*//6-31 + G* ZPE + MP2/6-31 + G*//6-31 + G* MP2/6-31 + G*//MP2/6-31 + G*

1

2

3

4

5

6

6.86 7.12 4.61 6.35

0.00 0.00 0.00 0.00

27.60 28.59 25.50

36.83 35.05

9.26 12.02 11.15 11.47

30.18 26.91 24.63 25.96

G. Raabe/J. MoL Struct. (Theochem) 313 (1994) 55-72

At a correlation coefficient of r = 0.990 there is a surprisingly linear relationship between those stretching vibrations to which the shorter silicon-nitrogen bond contributes significantly and the lengths of these bonds. The frequency of the asymmetric stretch of the N - S i - N segment of 1 (1169.6 cm l) is closer to the value of an S i = N double bond (1225.9 cm -1) than to that of an S i - N single bond (863.6cm-1). This situation might be compared with the carbonanalogue case, where the CN bond in H3N2CLi (C1) has a length of 131.3pm. The c a r b o n nitrogen single and double bond distances in H 3 C - N H 2 and H 2 C = N H are 145.4pm and 125.3 pm respectively. The CN bond in C1 is therefore 9.8% shorter than a typical carbon-nitrogen single bond, indicating that conjugative interaction in the N - X - N segment is significantly stronger for X = C than for X = Si. In addition, two types of bond separation reactions [13-15] (Schemes 4 and 5) were used to estimate the interaction of the formal X = N and X - N bonds in the N - X - N segments for X = C and X = Si. While reaction (2) is approximately isodesmic, (1) is clearly not. For the non-isodesmic (1) and the isodesmic (2) bond separation reactions [14],

59

energies of 50.4 kcal mol -l (1) and 26.2 kcal mol -l (2) were obtained for X = Si. The corresponding values for X = C are 58.8kcalmo1-1 (1) and 34.5kcalmo1-1 (2), indicating that interaction of the X = N and X - N fragments is energetically more favourable in the carbon case. To get an impression of the importance of correlation corrections for both reactions, all energies were recalculated using H F total energies. For X = Si the energies of bond separation are 48.9kcalmol -l for reaction (1) and 27.3kcalmol -l for reaction (2). The corresponding values for X = C are 54.5kcalmol -l (1) and 32.9 kcal mol-I (2). As expected, correlation corrections play a less important role in the case of reaction (2). Moreover, they seem to be more significant for the carbon compounds. The total energy of 5 is 12.0kcalmol -l higher than that of 2. The Si-N1 bond length in 5 is essentially equal to that of the S i - N bond in H 3 S i - N H 2, while shortening of the Si-N2 bond relative to the silicon-nitrogen bond in silylamine indicates partial double-bond character. Characteristic features of 5 are the small S i - N 2 - L i angle of 97.6 ° and a short Si-Li distance of 261.8 pm which is only slightly longer than the

H

I Y

/ \ H-X

Li

+ XH 4 + YH 3

-->

H2X=YH

+ H3X-YH 2 + Li-YH 2

(i)

\/ Y

I

H

H

I Y

/ \ H-X

Li

\ / Y

I H

+ XH 4 + 2YH 3

->

H2X=YH

+ H3X-YH 2 + H3Y.Li-YH 2

(2)

G. Raabe/J. Mol. Struct. (Theochem) 313 (1994) 55-72

60

Si-Li bond in H3SiLi (252.3 pm). (The sum of the isotropic van der Waals radius of silicon and the radius of lithium in the non-bonded state is 392 pm [16].) Both the relatively small Si-N2-Li angle and the short distance between the silicon and the lithium atom might indicate a bonding interaction between silicon and lithium. The characteristics of this structure are retained upon further optimization including MP2 corrections (vide infra). The Si-N-Li angle (92.4 °) and the Li-Si bond distance (252.7pm) decrease compared with the HF/6-31+G* optimized values. As in the case of 2, the HOMO of 5 is a molecular orbital of cr symmetry with significant s character widely located at the silicon atom. 3, which is 28.6 kcalmo1-1 higher in energy than 2, is formally related to 5 by a 1, 2 hydrogen shift. The Si-N2 bond length is 158.6 pm and thus only 0.8 pm longer than the double bond in HzSi=NH, and the frequency of the asymmetric stretching vibration of the N - S i - N segment (1248.2cm l) is close to the value for an Si=N double bond. The Li-N1 bond distance in 3 is larger than in lithium amide probably indicating a reduced back-donation of the nitrogen lone pair into an empty 2p orbital at the lithium atom. Elongation of both the Si-N2 and the Li-N1 bonds might be explained in terms of conjugative interaction between the lone pair at N1 with the Si=N2 double bond.

Li~ / H

Li

/H

/

H-Si

0

-

.~N-.H A"

H-Si

-IN..H 8

This assumption is further supported by shortening of the Si-N1 bond relative to the value for the silicon-nitrogen bond in H3SiNH2 by about 7 pm, which is too large to be explained by the different hybridization states of the silicon atom in silylamine (~ sp 3) and 3 (~ sp2). Hence, the Si-C single bond lengths in H3Si-CH3 (189.1 pm)

and H3C-SiH=CH2 (188.0pm), for example, differ by only 1.1 pm. All attempts to optimize a structure derived from 3 by a 180° rotation of the L i - N 1 - H 2 segment about the N2-Si axis resulted in 2. With an energy of 26.9 kcal mol 1 relative to 2, 6 was the least stable local minimum under consideration. 6 is formally related to 5 by a 1, 2 hydrogen shift, and at 14.9kcalmol 1 the energy difference between these two molecules is close to that between silianimine and aminosilylene. As a result of sp hybridization of N2, the Si=N2 bond is even shorter than in H2Si=NH and the frequency of the normal vibration widely located in this region (1369.7cm ~) exceeds that for the Si=N bond in HzSi=NH (1225.9cm-1). The essentially linear Si-N2-Li fragment allows effective backdonation of the nitrogen lone pair into an empty 2p orbital at lithium. The nitrogen atom is approximately sp hybridized and as a result, the Li-N2 bond is slightly shorter than in lithium amide. With one imaginary frequency in the spectrum of its normal modes 4 corresponds to a saddle point. The Si-Li distance in 4 (248.7 pro) is even shorter than in 5, and the lithium atom occupies a bridging position above the Si-N2 bond. In the search for further local minima, the stability of 1-3, 5 and 6 against deviations from planarity up to 20pm was checked. In all cases, the planar structures were recovered upon reoptimization. In order to test the post-HF stability of some of the obtained local minima, the structures of the four most stable isomers were reoptimized at the MP2/6-31 +G* level. Not only the main structural features of the molecules were retained in these calculations but also the calculated bond lengths and angles are not too different from the 6-31 + G* optimized values. Moreover, the relative energies obtained at this level (see Table 2) are quite similar to the MP2/6-31 +G*//6-31 +G* results. Table 3 Total energies of dimers DI and D2 (in hartrees)

6-31 + G*//6-31 + (3' MP2/6-31 + G*//6-31 + G*

D1

D2

-814.194778 -815.107898

8|4.188819 -815.115250

G. Raabe/J. Mol. Struct. (Theochem) 313 (1994) 55-72

H(~H1

D1 H2

H2

D2 i

N2

Si AI

H3

61

"~H3

N1

Li' HI'

Li'~

Li

H1 Li

(~

r2'

H3' N2' HI' Q_.)

H3'

Fig. 2. Dimers D1 and D2. Bond lengths in pm, bond angles in degrees. DI: r ( N 1 - S i ) = 16Yl, r ( N 2 - L i ) = 191.5, I N I - S i - N 2 = 124.1, LSi-N2-Li = 127.2, L N 1 - L i - N 2 ' = 161.6. D2: r(N1-Si) = 167.7, r(N2-Si) = 186.9, r(N1-Li) = 199.1, r(N2 Li') = 198.8, ~N1 Si N2 = 97.1, LSi-N1-Li = 160.4, LNi Li N2' = 165.6.

3.1.2. Dimerization

Since dimerization and oligomerization is a common phenomenon among organolithium compounds [17], the dimerization reactions of the most stable molecules 1 and 2 were studied. The total energies of the dimers (D1, D2), which have been optimized under constraint of Czv (D1) or Cs symmetry (D2), are given in Table 3, while the optimized structures are shown in Fig. 2. At 191.5 pm, the L i - N bonds in D1 are shorter than in 1 but the S i - N bond lengths are approximately the same in the monomer and the dimer (163.1pm). The N - L i - N segment encloses an angle of 161.6 ° and thus deviates significantly from linearity (see Fig. 2). At the 6-31 + G*//6-31 + G* level dimerization of 1 is exothermic with a reaction energy of JAccording to ref. 18 and unpublished results by the author the eight-membered ring (head-to-tail) dimer of H2C=C=NLi, is not only a local minimum but also significantly lower in energy than its isomer forming a four-membered ring [12]. At the 631G*//6-31G* level this energy difference is 17.6kcalmol -l. Moreover, optimization of a Dl-like dimer of 1,3-diaza-2phosphaallyllithium at the triple-zeta SCF level resulted in a minimum of D2h symmetry with L i - N bond lengths and N - L i N bond angles quite similar to those obtained for D1 [191.

-53.1kcalmo1-1. Inclusion of MP2 corrections yields an almost identical value of -53.8kcal mol -l. This value is somewhat less than the dimerization energy of a lithiated aminonitrile at a slightly lower level ( - 6 0 . 5 k c a l m o l - l ; MPZ/6-31G*//6-31G* [12]). At the uncorrelated level, D2, a formal dimer of 2, is 3.7 kcalmo1-1 higher in energy than D1. This value, however, changes to 4.6 kcal mol-1 in favour of D2 when MP2 corrections are included. Nevertheless, the dimerization energy of 2 yielding D2 is lower than that of 1 resulting in D1. Its energy of dimerization is -35.6 kcal mo1-1 at the H F level and changes to -44.2 kcalmo1-1 when correlation corrections are included in single point calculations. Bond lengths and angles in the molecular subunits of D2 are not too different from those in 2 and the monomers can still be easily recognized (see Fig. 2). Since the spectra of the normal modes of D1 and D2 were not calculated one cannot be sure that these structures are local minima. There are, however, lithium compounds for which dimers containing an eight-membered ring [18] are indeed minima [19]. 1 This, of course, does not mean that this is also the case for D1 and D2, and the changes of energy associated with their formation from the

G. Raabe/J. Mol. Struct. (Theochem) 313 (1994) 55-72

62

MS1A

MS1B

N2

H4 •

.,

N2 o

lIN0

II0nI ~ H5

N1

H4 ,@

N1

2 MS2B

MS2A

H3 ( ~ ( ~

H2 ~ N2

H4 Li

O

H4

mBm< Si

H5 N1

H5

Fig. 3. H 2 0 adducts of 1 and 2. Bond lengths in pm, bond angles in degrees• MS1A: r(Nl Si) = 162.3, r(N1 Li) = 201.8, r(Li O ) - 195.2, L N 1 - S i - N 2 - 110.4, L S i - N I - L i = 8 3 . 5 , ~/N2 Li O - 138.7. MS2A: r(Nl S i ) = 165.8, r(N2-Si) = 189.2, r(Nl Li) = 188.2, r(N2 Li) = 201•8, r ( L i - O ) - 194.3, LN1 Si N2 = 94.7, LSi-N1-Li = 96.2, / S i - N 2 - L i = 84.9, ~NI Li O = 136.3, /N2 Li O = 139.5. MSIB: r(N1-Si) - 162.2, r(N1 Li) =202•2, r(Li O) = 196.8, L!NI S i - N 2 = 110.2, ~Si N1 L i = 83.8, ZN1 Li O = 138.9. MS2B: r(N1-Si) = 165.9, r(N2 Si) - 188.7, r(N1 Li) = 188.1, r(N2 Li) - 202.0, r ( L i - O ) = 197.3, ZNI Si N2 = 94.6, ~/Si-N2 Li - 85.1, ~N2 Li N1 = 83.9, ~N2 Li O = 136.0, ~NI Li O - 140.1.

DS1

H4 5 ~

H2

DS2

1~

H4'@~-~ ~O'

, ~, H 5

H1

Ha

H5' k._2 H5'

Fig. 4. 1 . ( H 2 0 ) 2 and 2 . ( H 2 0 ) 2 . Bond lengths in pm, bond angles in degrees. DSI: r ( N 1 - S i ) = 162.4, r ( N 1 - L i ) = 2 1 0 . 2 , r(Li O) = 199.1, L N 1 - S i - N 2 = 111.7, LSi NI Li -- 84.4, LN1 Li O = 106.9, LO Li O' = 135.5. D S 2 : r ( N 1 Si) - 166.6, r(N2-Si) = 187.9, r(N2 L i ) = 204.6, r ( N l - L i ) = 205.2, r ( L i - O ) = 200.4, L N I - S i - N 2 = 95.9, ~/Si N 2 - L i = 89.0, I N I . - L i - N 2 = 80.1, LN2 Li O -- 122.2, ~ N l - L i - O = 92.2, L O - L i - O ' = 115.2, L N I - L i - N 2 = 80.1.

63

G. Raabe/J. Mol. Struct. (Theochern) 313 (1994) 55-72 Table 4 Total energies of adducts (in hartrees)

6-31 +G*//6-31 + G * MP2/6-31 +G*//6-31 + G *

MS1A

MS1B

MS2A

MS2B

DS1

DS2

-483.098572 -483.752579

-483.096571 -483.750308

-483.109170 -483.764012

-483.107552 -483.761979

-559.135092 -559.990464

-559.144899 -559.999993

corresponding monomers should be regarded as upper bounds to the 'true' values.

3.1.3. The possible influence of a solvent on the relative stabilities of 1 and 2 To estimate the influence of a lithium-complexing solvent on the relative stabilities of 1 and 2 in a supermolecule approach, calculations were performed for some hypothetical water adducts of the two most stable isomers. Calculations on monosolvated 1 and 2 were carried out under constraint of Czv and Cs symmetry. The obtained stationary points are shown in Figs. 3 and 4, while total energies are given in Table 4. In the case of 1, a planar structure (MS1A, Fig. 3) was found to be 1.4kcalmo1-1 lower in energy than a structure in which the planes defined by H3NzSiLi and H 2 0 are orthogonal to each other (MS1B, Fig. 3). The relative energy of the corresponding adducts of 2 (MS2A and MS2B, Fig. 3) is 1.3kcalmo1-1 in favour of MS2A. At the 6-31 +G*//6-31 + G * level the monosolvate of 2 is 6.7kcalmo1-1 lower in energy than that of 1 and this value changes slightly to 7.2 kcal tool -I when MP2 corrections are included. The relative energy of the solvated species is therefore almost the same as for the free molecules, and the first solvation energies are -19.0 kcalmo1-1 for both 1 and 2. Since in both 1 and 2, the lithium atom is already bonded to two nitrogen atoms, the exothermicity of the addition of the first solvent molecule is slightly lower than for such a molecule where the lithium atom is coordinated by a single atom [12,20]. Nevertheless, these values are surprisingly close to an average value of - 2 0 . 8 k c a l m o l 1 obtained from the solvation energies of five lithium compounds at the same level of precision [20]. Those authors concluded that, in general, the solvation energies of first-row lithium compounds LiX are quite independent of the nature of X. The values obtained for 1 and 2 support this notion.

In order to see whether the relative energy of 1 and 2 will be reversed when a second solvent molecule is added to the lithium atom, further calculations were carried out for DSI and DS2 (Fig. 4). Again the structures were optimized under constraint of C2v (DS1) and C s (DS2) symmetry. The additional amounts of energy gained by addition of the second water molecule are 16.7kcalmo1-1 for 1 and 15.5kcalmol i for 2. With and without MP2 corrections, the relative energies of DS1 and DS2 are 6.0 and 6.2kcal mol 1, respectively. Thus the energy difference between the two most stable isomers seems to be preserved when their lithium atoms are complexed by two water molecules. A closer inspection of the optimized structures, however, reveals that, in both DS1 and DS2, hydrogen atoms of the complexing water molecules might interact with the nitrogen atoms. This, of course, is not to be expected when the solvent is diethyl ether or tetrahydrofuran. The second solvation energies calculated with water as a model solvent might therefore be lower than the 'true' values. In DS1 the shortest N - . . H - O - H distances are 296.8pm. While there are four such distances of equal lengths in DS1, there are two relatively short N 1 - . . H - O - H distances of 225.1pm in DS2, and the N 2 . . . H - O - H bonds are much longer (347.9pm). Thus one might speculate whether the artificial lowering of the second solvation energy is more important in DS2 and that there is a chance that the energy difference between 1 and 2 might be reduced or even reversed under the influence of a lithium-coordinating solvent.

3.2. H3N2Si- ( A I - A 4 ) Total and relative energies of anions A1-A4 are listed in Tables 5 and 6, and the optimized structures are plotted in Fig. 5.

64

G. Raabe/J. Mol. Struct. (Theochem) 313 (1994) 55-72

Table 5 Total energies of A1-A4 (in hartrees)

6-31 +G*//6-31 +G* MP2/6-31 +G*//6-31 +G* ZPE +MP2/6-31 + G*//6-31 + G*

A1

A2

A3

A4

-399.535067 -399.989499 -399.952464

-399.555076 -400.008402

-399.542106 -399.996375

-399.555141 -400.008738 399.967343

Among all isomers of general formula H3NzSi considered in this study A4 is lowest in energy. At 164.9pm the length of the Si-N1 bond almost equals the average value of that of the lengths of an S i - N single and Si=N double bond. Different

bond in H 3 S i - N H . In the carbon analogue of A1 the N - C bond (131.1pm) is 8.5% shorter than in H 3 C - N H (143.3pm). The energy change associated with the isodesmic bond separation reaction shown in Eq. (4) is

H

I

Y

/ H-X

\

-

+

XH 4

->

H2X=YH

+

(4)

H3X-YH

Y

I

It

from planar saddle point A2, the N H 2 group in A4 is slightly pyramidalized with an inversion barrier as low as 0 . 2 k c a l m o l - l . 2 One barrier toward rotation about the Si-N2 bond amounts to 7.8 kcalmol 1. A1 is 12.1 kcalmol -I higher in energy than A4. The energy of the reaction AI + Li + -~ 1

29.0kcalmo1-1 (6-31 + G*//6-31 + G * ) for X = Si and 54.0kcalmo1-1 for X = C. This indicates a much more effective interaction between the X = C and X C segments when X is carbon. A2 and A3 represent saddle points on the H3N2Si surface, each one having one imaginary frequency in the spectrum of its normal vibrations.

(3)

is 179.2kcalmol -l and thus only 5.9kcalmol -l lower than the energy associated with the formation of lithium amide from N H 2 and Li +. Since the L i - N bond in 1 is much longer than in lithium amide, the relatively high energy of reaction for (3) is probably due to the presence of a second Li N bond in 1. The S i - N bond in A1 (162.5pm) is only 4.7 pm longer than the double bond in H2Si=NH while it is 3.2pro or 1.9% shorter than the S i - N 2The inversion barrier in ammonia is 5.0 kcal mol-I at the same level of accuracy (experimental value: 5.8 kcal mol i [21].

3.3. H~P2SiLi (P1-P6, PI'-P6' ) Replacement of the nitrogen atoms in 1-6 by phosphorus, adjusting the bond lengths to Table 6 Energies of AI-A4 relative to the most stable isomer (in kcalmol t)

6-31 +G*//6-31 +G* MP2/6-31 + G*//6-31 + ( 3 ' ZPE + MP2/6-31 + G*//6-31 + (3*

A1

A2

A3

A4

12.60 12.07 9.34

0.04 8 . 1 8 0.00 0 . 2 1 7.76 0.0 0.0

65

G. Raabe/J. Mol. Struct. (Theochem) 313 (1994) 55-72

A1

(i) ~1 !

A2

A

si H3

N1

S

~

N1

( ~ ) H2

Si

H3

H2 ~ H 1

Fig. 5. Stationary points of sum formula H3SiN2. A1 and A4 are minima, A2 and A3 saddle points. Bond lengths in pm, bond angles in degrees. AI: r(N1-Si)= 162.5, LN1-Si-N2 = 130.7. A2: r(N1-Si)= 165.0, r(N2-Si)= 177.0, ~N1 S i - N 2 - 104.0. A3: r(N1-Si) = 164.9, r(Si N2) = 183.9, LN1-Si-N2 = 105.0. A4: r(N1-Si) = 164.9, r(Si N2) = 177.5, LN1-Si-N2 = 103.7.

reasonable values followed by optimization, and imposing the same symmetry constraints as in the case of the nitrogen compounds resulted in the structures shown in Fig. 6 (P1-P6). The corresponding total and relative energies are given in Tables 7 and 8. Calculation of the normal vibrations, however, revealed that none of these structures corresponds to a local minimum. While there is a single imaginary frequency in the spectra of P2-P4 and P6, there are two in the spectra of the normal vibrations of P1 and PS. Thus the local minima of Cs and Czv symmetry obtained for the nitrogen compounds have no counterparts among the phosphorus species. Taking into account the higher inversion barrier at phosphorus than at nitrogen (vide infra), deviations from planarity in case of the phosphorus compounds are not surprising. This high barrier to inversion at phosphorus causes marked differences between the L i - N and the Li-P bonds. These differences might be exemplified by comparing H2NLi with H2PLi. In HzNLi interaction of the nitrogen lone pair with an empty 2p orbital at lithium is obviously sufficient to flatten the molecule, resulting in an L i - N linkage with partial double bond character. Things are

different in the case of H2PLi. This molecule is distinctly pyramidalized. Forcing the molecule into a plane reduces the Li-P bond length to 224.4 pm, but the energy of the planar structure is still 5.5kcalmo1-1 above that of the optimized molecule. Thus, reoptimization of the H3SiP2Li isomers without constraint of symmetry conditions yielded the structures shown in Fig. 7 (PY-P6r). The corresponding total and relative energies can be found in Tables 9 and 10. In this way planar P1 rearranged to PY, where the lithium atom as well as the hydrogen atoms lie significantly above or below the plane defined by the P - S i - P segment. Among all isomers of general formula H3P2SiLi, P1 r is the most stable. It is 8.6kcalmo1-1 lower in energy than planar P1. As to be expected the Si-P bonds are slightly longer in P1 ~ (214.5pm) than in P1 (212.7pm). The P-Si bonds in P1 ~ are 4.0% longer than the Si=P bond in silanephosphimine H2Si=PH (206.2pm) and 5.6% shorter than the Si-P bond in H3Si-PH 2 (227.2pm). At 263.0pm, the Li-P bonds are 25.2pm longer than that in LiPH2 (237.8pm). It is interesting to note that the distance between the lithium and the silicon

G. Raabe/J. Mol. Struct. (Theochem) 313 (1994) 55-72

66

H1

P2

P1

Si Si

H2

H1

H3

P1

Li

Li

P3

( )II1

1DA

Li

P1 P2 Si

H2

H3

~I3

P5

~

Li

H3

P6

P2

Si H1 P1 ~

H3

P1

( ~ H2

Li H2

Fig. 6. Stationary points of sum formula H3SiPzLi obtained under constraint of Czv (PI) and C~ ( P 2 - P 6 ) symmetry. None of them corresponds to a local minimum. Bond lengths in pm, bond angles in degrees. PI: r(P1 L i ) = 247.3, r(Si P 1 ) - 212.7, LP1 Si-P2 = 109.3, LLi-P1-Si = 80.8, LP1-Li-P2 = 89.1. P2: r(Pl Li) = 236.5, r(P1 Si) = 214.7, r(P2-Li) = 249.8, r(P2-Si) = 239.7, L P l - S i - P 2 = 88.8, L P 1 - L i - P 2 = 81.7, LLi P1 Si = 99.8, P3: r ( P l - L i ) = 229.4, r ( P l - S i ) = 216.1, r(P2-Si) = 208.8, LLi-P1-Si = 128.2, LP1-Si P2 = 135.4, P4: r(P1-Si) = 208.9, r(Si-Li) = 250.0, r(Si-P2) = 240.2, r(P2 Li) = 249.3, LP1-Si-P2 = 108.6, LP1-Si-Li = 169.7, LSi-Li-P2 = 57.5. PS: r(P1 Si) = 218.6, r(P2 Si) = 219.2, r(P2-Li) = 229.2, L P I - S i - P 2 = 102.5, LSi-P2-Li = 123.7. P6: r(P1 Si) = 219.3, r(Si--P2) = 203.5, r(P2-Li) = 232.0, LP1-Si-P2 = 133.3, S i - P 2 - L i = 100.2.

atoms (257.1pm) is only about 5pm longer than the Si-Li bond in H3SiLi (vide supra). A normal vibration at 615.2cm -1 is assigned to stretching of the silicon-phosphorus bond in PI'. This is closer to the value of 651.5cm -1 for the Si=P bond in HESi=PH than to that for the Si-P single bond in H3SiPH 2 (480.3 cm-l).

Recently, the synthesis and solid-state structure of the lithium salt of a highly substituted 1,3-diphospha-2-silaallyl anion (D, Scheme 1) was reported [2]. Although solid-state data and calculated ab initio structural parameters are not directly comparable to each other at a highly precise level, it is worth mentioning that the reported solid-state structure of the PzSiLi

G. Raabe/J. Mol. Struct. (Theochem) 313 (1994) 55-72

67

Table 7 Total energies of P1-P6 (in hartrees)

6-31 +G*//6-31 +G* MP2/6-31 +G*//6-31 +G*

P1

P2

P3

P4

P5

P6

-979.551719 -979.891823

-979.557813 -979.897483

-979.541501 -979.878534

-979.526633 -979.869276

-979.517148 -979.852058

-979.510783 -979.855712

segment of D is much more similar to that in P1 than in PI'. Thus, for example, the LiP2Si moiety in D is not only planar, but also the experimentally determined P - S i - P angle of 104.9° is much closer to the corresponding value of 109.3° in P1 than to the P - S i - P angle of 125.8° in P1 ~. Moreover, at 211.4pm the experimentally determined Si-P bond length is not significantly different from the one calculated for P1 (212.7 pm). Probably due to the presence of two complexing diethyl ether molecules in the solid-state structure of D, the experimental values for the Li-P bond lengths of 263.9-267.0pm not only exceed the theoretical value for P1 (247.3pm) by 7.3% but are also longer than the experimental bond lengths in lithiophosphanes [2]. The assumption that the difference in Li-P bond lengths between D and P1 is indeed due to the presence of two solvent molecules is supported by the results obtained for the nitrogen species, where the L i - N bond length in 1 increased by 6.8% when two water molecules are added to the lithium atom. As in the case of the nitrogen compounds bond separation, reactions were used to compare the interaction of the X=P and X - P segments for X = C and X = Si. For X - - S i the changes of energy associated with reaction (1) are 24.2 and 32.5kcalmo1-1 at the 6-31+G*//6-31 +G* and MP2/6-31 + G*//6-31 + G* levels respectively. The corresponding values for reaction (2) are 12.6 without and 18.5kcalmol i with correlation corrections, respectively. For X = C, reaction (1)

yielded 18.7kcalmo1-1 (6-31 + G*//6-31 + G*) and 27.8kcalmo1-1 (MP2/6-31 + G*//6-31 + G*), while 7.2kcalmo1-1 (6-31 + G*//6-31 + G*) and 13.7kcalmol 1 (MP2/6-31 + G*//6-31 + G*) were obtained from reaction (2). In the case of the nitrogen compounds, 2 (Cs) was found to be about 7kcalmo1-1 lower in energy than 1 (Czv). When nitrogen is replaced by phosphorus but the same symmetry constraints are enforced, P2 (Cs) is still 3.6kcalmo1-1 lower in energy than P1 (C2v). However, when the structures are reoptimized without symmetry constraints the relative energy of the resulting two isomers is interchanged in that PI' is now 4.5kcalmo1-1 lower in energy than P2' and inclusion of zero point vibrational energy does not alter this energetic order. This is in keeping with other theoretical results which, in contrast to the nitrogen-analogue case, predict planar HzSi-PH to be 13 [22] or ll.9kcalmol 1 [23] lower in energy than its conjugated silylene equivalent (H-Si-PH2). Since the lone pair of the phosphorus atom has a stronger s character than that of the nitrogen atom this is most probably due to a lower stabilization of the silylene by a phosphino group compared to that by an amino group [24]. While the structural differences between P1 and P1 ~ are striking, they are less significant between P2 and P2'. Consequently the energy difference between the latter isomers is less than 0.6kcalmo1-1. Although P2 ~ deviates significantly from planarity, the bond lengths are almost identical

Table 8 Energies of P1-P6 H3SiP2Li relative to the most stable isomer (in kcal mo1-1)

6-31 + G*//6-31 + G* MP2/6-31 + G*//6-31 + G*

P1

P2

P3

P4

P5

P6

3.82 3.55

0.00 0.00

10.24 11.89

19.57 17.70

25.52 28.50

29.51 26.21

68

G. Raabe/J. Mol. Struct. (Theochem) 313 (1994) 55-72

P2' I S i

P

I

~

H3

Li

~

Li

P3 '

H2

P4' H1

H2

P5'

S

i

P6'

~

H3

~) H3

H1

P2 Si

~ H 3 P1

@~HH2P2 H2 Li

Fig. 7. Stationary points of sum formula H3SiP2Li obtained in geometry optimizations without symmetry constraints. All molecules except P4' are local minima. Bond lengths in pm, bond angles in degrees. PlY: r(Pl Si) = 214.5, r(Si-Li) = 257.1, r(P1 Li) = 263.0, LP1-Si-P2 = 125.8, LP1 Li-P2 = 93.1, LSi P I - L i = 64.2. P2~: r(P1 Si) = 215.0, r(P1 Li) = 236.4, r(P2-Si) - 239.1, r(P2 Li) = 249.7, LSi-P1-Li = 96.5, LSi-P2-Li = 87.2, LP1 Li-P2 = 81.8, LP1 Si-P2 = 88.9. P3r: r(P2 Si) - 209.1, r(P1 Si) = 218.7, r(P1 Li) = 239.3, LP1 Si-P2 = 135.5, LSi P I - L i = 90.0. P4': r(P1 Si) = 208.9, r(Si P2) = 239.9, r(Si-Li) = 250.8, r(P2-Li) = 249.2, LP1-Si-P2 = 108.0, LP1-Si L i = 168.0, LP2-Si Li = 61.0, :Si-Li P2 = 57.3. P5': r(P1-Si) - 228.1, r(P2-Si) - 214.3, r(Si-Li) = 260.9, r(P2 Li) = 236.2, LP1 Si-P2 = 110.8, LP1-Si-Li = 148.8, LSi-P2 Li = 70.6, LSi-Li P2 = 50.8. P6': r(P1-Si) = 228.1, r(P2-Si) - 203.5, r(P2-Li) = 233.5, /P1--Si-P2 - 131.8, LSi P2 Li = 99.0. Table 9 Total energies of P1 ~ P6' (in hartrees)

6-31 +G*//6-31 + G * MP2/6-31 +G*//6-31 + G * ZPE +MP2/6-31 +G*//6-31 + G *

PI'

P2'

P3'

P4 ~

P5'

P6'

-979.558060 -979.905533 -979.875205

-979.557946 -979.898362 -979.866497

-979.547380 -979.888441 -979,858273

-979.526672 -979.869371

-979.540379 -979.879533 -979.848579

-979.540578 -979.884033 -979.853212

69

G. Raabe / J. M ol. S truct. ( Theochem ) 313 (1994) 55-72 Table 10 Energies of P I ' - P 6 ' relative to the most stable isomer (in kcal tool -l)

6-31 + G*//6-31 + G* MP2/6-31 + G*//6-31 + G* ZPE + MP2/6-31 + G*//6-31 + G*

PI'

P2'

P3'

P4'

P5'

P6'

0.00 0.00 0.00

0.07 4.50 5.46

6.70 10.73 10.62

19.70 22.69

11.09 16.32 16.71

10.97 13.49 13.80

to those in P2 and the same holds for the P - S i - P and P - L i - P angles. The fact that the perpendicular distance of the lithium atom from the plane defined by the P - S i - P segment amounts to 76.5pm is, therefore, predominantly due to the smaller Si-P-Li angles in P2' compared with P2. The formation of P2' from its formal constituents LiPH2 and HPSi yields 56.5kcalmo1-1 and is, therefore, somewhat less exothermic than the formation of 2 from HNSi and LiNH2. P3 ~ is 10.7kcalmo1-1 higher in energy than PI', while its total energy is 6.2kcalmol -l below that of P3. In P3' the H3SiP 2 segment is almost planar and the bond lengths and angles of this fragment are not too different from those in P3. The Li-P bond in P3', however, is almost perpendicular to the plane defined by the remaining molecule, and this bond is about 10pm longer than in P3. In the lattermentioned molecule, where the lithium atom is forced into the plane of the heavy atom skeleton, the Li-P linkage might be described as a bond between a lithium l s orbital and a hybrid with significant s character on phosphorus, while the two non-bonding electrons at phosphorus occupy a 3p orbital. Back donation from this p-type lone pair into an empty p orbital at lithium would play a role in this structure. In P3' the coordination sphere of phosphorus is characterized by bond angles which are all close to 90 °. Therefore, the Li P bond in this molecule

can be described as a bond between a lithium l s and a phosphorus 3p orbital, while the two non-bonding electrons now occupy the phosphorus 3s AO. This is a result of the well-known reluctance of the s electrons of heavier elements to participate in a bonds, and hence accounts for the structural and energetic differences between P3 and P3'. The next species on the energy scale is P6' which is 13.5kcalmol I higher in energy than PI'. The inversion barrier at the phosphorus atom P1 in P6' is equal to the energy difference between P6 and P6' (17.8kcalmol-1). Owing to interaction of the phosphorus lone pair with the Si-P2 bond, it is 17.1kcalmol 1 lower than the inversion barrier in PH3 (experimental values: 31.8kcalmo1-1 for PH3 and 22.0kcalmo1-1 for P(CH3) 3 [25]). Again the different hybridization states of the phosphorus atom P1 in P6 and P6' account not only for the different bond lengths in the Si-P1-H1-H2 segments of both molecules but also for the lower total energy of

P6'. With an energy of 16.3 kcal mol 1 above PI', P5' is the least stable local minimum considered. Its structure is featured by a lithium atom in a bridging position above the Si-P2 bond. The metal atom lies 121 pm above the plane defined by the P1-Si-P2 segment, and at 236.2pm the Li-P2 bond length is quite similar to the Li-P1 bond in P2 t. The Li-P2 bond in P5' is therefore among the

Table 11 Total energies of PA1-PA6 (in hartrees)

6-31 + G*//6-31 + G* MP2/6-31 + G*//6-31 + G* ZPE + MP2/6-31 + G*//6-31 + G*

PAl

PA2

PA5

PA6

-972.118837 -972.450736 -972.422498

-972.104889 -972.435169 -972.406227

-972.101243 -972.430806 -972.402053

-972.088301 -972.424981 -972.396208

G. Raabe/J. Mol. Struct. (Theochem) 313 (1994) 55 72

70

PAl

PA2

HI

P1

(~ H2 H2

H3

PA6

PA5 H3

~ S i

~ H1

H3

~

si P1

H2 Fig. 8. Stationary points of sum formula H3SiP 2. All molecules are local minima. Bond lengths in pm, bond angles in degrees. PAl: r(P1 Si)=213.2, LP1-Si P2=138.8. PA2: r(P2-Si)=235.0, r(Pl-Si)=214.1, LP1 Si P2=100.9. PA5: r(Pl Si)=214.7, r(P2-Si) = 235.1, LP1-Si-P2 = 105.2. PA6: r(P1-Si) = 201.7, r(P2 Si) = 231.0, ZP1-Si-P2 = 129.9.

shortest bonds between phosphorus and lithium obtained in this study. The distance between the silicon and the lithium atoms (260.9 pm) is slightly longer than in P1 '. In saddle point P4', where the lithium atom also occupies a bridging position, the Si-Li distance (250.8 pm) is even shorter.

3.4. H3PeSi Removing an Li+ cation from P I ' - P 6 ' yielded the starting geometries for the anions of general formula H3P2Si-. PI' and P3 ' yielded the same anions (PAl) and the same is true for P2' and P4' which resulted in PA2. In this way four stationary points were located which are shown in Fig. 8, while total and relative energies are listed in Tables 11 and 12. All optimized structures correspond to local minima. Among all H3P2Si- anions, planar PAl is lowest in energy. It is of special interest that the H3PzSi segment of PI' flattens when the Li + cation is removed. Provided the planarity of the PzSiLi fragment in D is not enforced by the bulky substituents, it is reasonable to

describe that compound as a lithium salt. The length of the SiP bond in PAl is closer to the value for the SiP bond in HzSi=PH than to that in silylphosphine. The frequency of the asymmetric stretch of the P Si-P segment (633.6cm 1) is also quite close to the value for the Si=P bond in silanephosphimine. PA2, one possible silylene derived from PAl, is more than 9kcalmo1-1 higher in energy than latter compound, and its shorter SiP bond is 3.8% longer than that in HzSi=PH. Singlet PA6 is 16.2kcalmol -I higher in energy than PAl. The SiP bond in PA6 (201.7pm) is significantly shorter than the corresponding bond in phosphasilanimine. It is quite close to the SiP bond in HPSi (204.3pm) but still about 5% longer than the SiP bond in Table 12 Energies of PAI-PA4 relative to the most stable isomer (in kcal tool 1) PAl 6-31+G*//6-31+G* MP2/6-31 +G*//6-31 + G* ZPE+MP2/6-31+G*//6-31+G*

PA2

PA5

PA6

0.00 8 . 7 5 11.04 19.16 0.00 9 . 7 7 12.51 16.16 0.00 10.21 12.83 16.50

G. Raabe/J. Mol. Struct. (Theochem) 313 (1994) 55-72

71

Table 13 Total and harmonic zero point energies (co) of auxiliary compounds (in hartrees)

Li÷ CH 4 NH 2 NH3 H2NLi H20 H2C=NH H3C-NHH2C-NH2 HzNLi • NH 3 H3N2 C- a H3 N2 C Lib SiH 4 H3SiLi c H3CSiH 3 PH2 PH3 HNSi HzSi=NH H3Si-NH H3Si-NH2 H2 PLi H2C=SiHCH3 H2C=PH H3C-PHH3 C-PH2 HPSi H 2Si=PH H3 Si-PHH3Si-PH2 H2PLi • PIt 3 H3P2C d H3P2CL i e

6-31 + G*//6-31 + G *

MP2/6-31 + G*//6-31 + G *

e0

-7.235536 -40.195668 -55.518084 -56.189499 -63.050438 -76.017743 -94.032666 -94.543255 -95.214173 -119.27437 f - 148.466384 - 155.998954 -291.225881 -298.081041 -330.273094 -341.852882 -342.448753 -343.922375 -345.055002 -345.659810 -346.287157 - 349.322315 -368.088681 -380.289768 -380.878841 - 381.487435 -630.154336 -631.335319 -631.961669 -632.536482 -691.789501 -721.023824 -728.484933

-7.235989 -40.338494 -55.711170 -56.365904 -63.242201 -76.211278 -94.329545 -94.868567 -95.523167 -119.646727 - 148.951192 - 156.484156 -291.319224 -298.185903

0.047632 0.019824 0.036984 0.025044 0.022910 0.043201 0.050674 0.068803 0.065485 0.047563 0.051842 0.033302 0.024771

-341.972453 -342.564420 -344.186188 -345.322917 -345.939636 -346.550730 - 349.449671

0.013759 0.026156 0.015068 0.031551 0.038153 0.053549 0.017129

-380.541464 -381.139163 - 381.740606 -630.358706 -631.545251 - 632.175562 -632.742409 -692.036490 -721.400786 -728.873055

0.036325 0.046257 0.058519 0.007469 0.026618 0.033962 0.045431 0.045617 0.034804 0.037839

aCarbon analogue of A1. bCarbon analogue of 1. CConventional C3v structure (distorted tetrahedral). At the MP4SDTQ/6-3 IG**//6-31G* level another local minimum with an inverted C3v geometry is 2.4kcalmol -l lower in energy [26]. dCarbon analogue of PAl. eCarbon analogue of P1 r. Like PI' the molecule is non-planar and the energy difference between the planar and the non-planar structure is 12.3kcalmol I. fRef. 20. HSiP (192.4pm). 3 Again the changes of energy a s s o c i a t e d w i t h r e a c t i o n (4) w e r e c a l c u l a t e d . F o r Y = P a n d X = Si t h e e n e r g y o f r e a c t i o n is 30.0kcalmo1-1, while the value for X=C is 3 1.9 k c a l m o l - 1. 3The HPSi molecule is not linear and the hydrogen atom almost bridges the SiP bond (< H P-Si = 70.6 °, HF/6-31 + G*).

Acknowledgements The author gratefully acknowledges the computing time for calculations on the SNI-S600/20 provided by the Land Nordrhein-Westfalen (Project No. P104) and the generous technical s u p p o r t b y M r . E. H e y n e a n d M r . D . a n M e y o f the Rechenzentrum der RWTH Aachen.

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G. Raabe/J. Mol. Struct. (Theochem) 313 (1994) 55 72

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