Electronic Structures of
S4N,, AsqS4,and
As4Se,
The Journal of Physical Chemistry, Vol. 82, No. 79, 1978 2121
Electronic Structures of Tetrasulfur Tetranitride, Tetraarsenic Tetrasulfide, and Tetraarsenic Tetraselenlde, and Their Anionk Species Karuyoshi Tanaka, Tokio Yarnabe, Akitorno Tachibana, Hlroahi Katott and Kenichi Fukui" Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyofo University, Kyoto 606, Japan, and College of General Education, Nagoya Unherslty, Nagoya 464, Japan (Received May 8, 1978)
The electronic structures and vertical electron affinities of SIN4, As4S4,and As4Se4are examined using semiempirical INDO-type ASMO-SCF calculations. According to the results, S4Nihas an unexpectedly lower energy than S4N4. This is caused by the unusual divalencies of nitrogen atoms in SIN4. Aa4S[ and As4Se4also have lower energies than As4S4and &See, respectively. Analyses of the patterns of the lowest unoccupied molecular orbitals (LUMO) of each neutral species showed the geometrical stability of S4N4compared with As4S[ and As4Se[. This encouragesus to believe that SIN4 may become a useful electron-acceptor or trapping center in solid state materials.
Introduction Since its first preparation in the preceding century,l tetrasulfur tetranitride, S4N4, has gained gradual importance as a starting material of various sulfur-nitrogen compounds including the most striking derivative, polymeric sulfur nitride, (SN),,2 which has had an impact in the field of solid state science in that it shows a metallike electrical conductivity and even becomes a superconductor a t ca. 0.3 K.4 On the other hand, the problem of the geometry of S4N4itself has attracted many investigators mainly because of its peculiar atomic arrangements. Up to the present, a geometry of D2d symmetry with four coplanar nitrogen atoms as shown in Figure 1A has been accepted by X-ray crystallography,5MO calculations,6and orbital interaction consideration? The electronic Structure of S4N4in this geometry has also been reported with the use of various MO calculations>s from which a lot of informations concerning this material has been supplied so far. It should be noticed, however, although not pointed out explicitly in the above reports, that the orbital level of the lowest unoccupied molecular orbitals (LUMO) of S4N4are remarkably negative, suggesting that S4N4is an electron deficient species. In fact, Q4N4is easily reduced to a radical anion by alkali metals in the liquid phase affording a nine-line ESR spectrumg and even forms a A possibility for tetraanion salt such as (Na+)4[S4N4]4-.10 the existence of the anionic species of SIN4 has been mentioned only qualitatively on the basis of extended Huckel MO calculationsoB8Therefore, it would he of interest to discuss the electronic structure of S4N4from this viewpoint and the possibility of the stabilizations of the anionic species in detail. Meanwhile, the geometries of molecular arsenic chalcogenides, As4X4(X = S or Se), proposed by X-ray diffraction analyses1' as shown in Figures 1B and 1C are also of D 2 d symmetries but with coplanar chalcogen atoms, being different from that of S4N4. The electronic structures of As4X4molecules have been investigated by Chen12 and Salaneck et all3 mainly with respect to the valenceband densities of states. Since there have been neither experimental reports nor theoretical studies on the anionic species of As4X4except for the analysis of the electronic structure of As4S4-by the present authors14in the course of the study on the electron-trapping center in As2S3glass, it is also interesting to examine the electronic structures College of General Education, Nagoya University.
* Address correspondence to this author at Kyoto University. 0022-3654/78/2082-212I$Oi .OO/O
TABLE I: Slater Exponents (er)and Valence State Ionization Potentials (I,) of AO's and Electron Repulsion Integralsa for As and Se .trb I,C (rrlrrId ( rr' I rr' )" As 4s 2.2360 17.952 9,234 (4s4p14s4p)= 0.370 4p 1.8623 9.187 7.684 ( 4 ~ 4 ~ ~ 1 4= ~0.300 4~') Se 4s 2.4394 20.838 9.046 (4s4p14s4p)= 0.540 4p 2.0718 10.784 7.369 (4p4p'14p4p') = 0.540f The units are shown in eV for I,, (rrirr), and (rr'lrr'). Reference 19. Reference 20. (rrlrr) = I, - E,.15 E, denotes the valence-state electron affinity of A 0 r. For values of E,, see ref 21. e Reference 22. f Assumed.
of As4S4and As4Se4and the energetic stabilities of their anions parallel with those of S4N4. In the present paper, we try to discuss systematically the electronic structures of S4N41As4S4,and As4Se4including their vertical electron affinities on the basis of the MO theoretical treatment. The bond energies and the frontier orbital patterns of the species are extensively studied. It is deduced that S4N4-has an unexpectedly lower energy than S4N,. This feature of S4N4-is more remarkable than As4S4-and As4Se4-that also have lower energies than As4S4and As4Se4,respectively. The cause of these stabilities is investigated by analyzing the atomic and the interatomic contributions to the total energies of the monoanions. The geometric stabilities of the monoanions are also discussed and the utility of S4N4as an electron acceptor or trapping center in solid state materids is suggested.
Method of Calculation The calculations are performed with the semiempirical INDO-type ASMO-SCF method for valence electrons.16 For open-shell systems, the unrestricted Hartree-Fock (UHF) treatment is adopted. Spin contamination inherent in this scheme is excluded by Lowdin's projection operator method16 simplified by Amos and Snyder.17 The MO method employed here has given fairly reasonable results for the shapes of molecular orbitals and the electronic structures of disulfur dinitride, S2N2,and its deformed geometries at the initial stage of polymerization to (SN),ls as well as those of As4S6and As4S4which are molecular analogue models in amorphous As2& glass.14 Parameters adopted for S and N are the same as those in ref 15,and those for As and Se are listed in Table I. The contributions from 3d atomic orbitals in the case of S4N4has been reported to be fairly small and not to play an imQ 1978 American
Chemical Society
2122
The Journal of Physical Chemistry, Vol. 82,No. 19, 1978
Tanaka et al.
TABLE 11: A 0 Densities and the Atomic Net Charges of S4N4,As,X4, and Their Monoanions S4N4
S4N4-
1.651 1.426 0.923 1.291
S
Px PY
Pz
1.651 0.923 1.426 1.291
S
Px PY Pz
Sl , z
4
1.645 1.675 0.906 1.298
As4
s,
A 0 Density A%,* 1.749 1.029 0.667 1.098
4
1.749 0.667 1.029 1.098
1.569 1.421 1.416 1.378
1.737 1.660 1.660 1.401
+0.476
Atomic Net Charge As,,, t 0.456 As,,, t 0.456
N 1.567 1.391 1.391 1.360
S
Px PY Pz
+0.709 t0.709
SlJ s3,4
t 0.593
1.768 1.101 0.674 1.121
1.764 1.018 0.705 1.131
1.736 0.901 1.030 1.082
1.764 0.705 1.018 1.131
1.746 1.656 1.666 1.476
1.801 1.617 1.617 1.347
t0.336
t 0.382
-0.784
-0.456
As4Se,-
As,,,
As,,,
1.782 1.043 0.698 1.143 1.752 0.924 1.011 1.084
Se
t 0.251
t0.382
S
portant role in the molecular descriptiona6 Similar conclusions have been reached on SO?-, SF6,23and various sulfur compounds of different valence states.24 Hence we neglect the d atomic orbitals of S, Se, and As in the present calculation. The total energy is partitioned into the one center term and the two center interatomic parts26in order to analyze the atomic (EA)and the interatomic contributions (EAB) to the total energy as follows:
r
h4Se4
S
N -0.709
AS,,
1.664 0.981 1.439 1.324
h4S4-
1.800 1.639 1.636 1.457
As,,, As,,,
t 0.334 t 0.229
Se -0.543
-0.382
-0.531
(A)
r'(#r) Y4
PrrJ) [ (rrlr'r 3 - 3 trr Irr 9I I t 1)
on A on B
EAB= 2 C C PrsHrs r
on A onB
s
on A on B
C C (Pr,)2(rrIss)+ C C (Nr - Prr)(Ns - Ps,)(rrlss) r s r s t 2) where E,"" A denotes the summation over all the valence 72
atomic orbitals on atom A, P,, and H,, are the bond order and the core-resonance integral, respectively, between the rth and the sth atomic orbitals. 1, is the valence state ionization potential of the rth atomic orbital, Nr is the number of the valence electrons occupying the rth atomic orbital in the valence state, and (rrlss)and (rrlrr? are the two-center Coulomb and the one-center exchange repulsion integrals, respectively. In eq 2, the first term is designated as the core-resonance term, the second term as the exchange term, and the third term as the electrostatic term. In the UHF treatment, (P,,)2 in eq 1 and Pr, in eq 2 should be replaced by 2 ( P r p ) 2+ 2(Pr,.,P)2and P,," + PrSp, respectively, where, e.g., p,," denotes the a spin density matrix element between the rth and the sth atomic orbitals. Results and Discussion The geometries employed in the calculations for the neutral species have D2d symmetries as shown in Figures 1A-C. However, one is obliged to choose lower symmetries for the monoanions owing to the degeneracy of two LUMO's of the neutral species as is described later. Thus
(C)
Figure 1. Vlew of the molecular structures of S4N4(A), As4S4 (B), and As,Se4 (C).
we tentatively employ here CZvgeometries for the monoanions under slight deformation due to the Jahn-Teller effect as shown in Figure 2, where each atomic position shifts 0.025 A along the arrow. These directions of the distortions are determined so that either of the two LUMOs of the neutral species might stabilize when it is occupied with one or two electrons. The calculated atomic orbital densities and the atomic net charge are listed in Table I1 and the frontier orbital patterns illustrated in Figure 3. The E A and E m values of the S4N4and As4X4series from eq 1 and 2 are shown in Tables 111-V, where the positive values mean a repulsive energy. In Table VI are shown the vertical electron affinities defined as the difference between the total energy of the neutral species and that of the anionic one. For reference are also shown those for the dianionic species of the DZdstructure in the triplet state. Throughout the
The Journal of Physical Chemlstry, Vol. 82, No. 19, 1978 2123
Electronic Structures of S4N4,As4S4, and As4Se4
TABLE 111: Atomic ( E A ) and the Interatomic Energies ( E A B ) in S4N4and S4N4-(in eV)
E m exchange
electrostatic
total
0.248 -0
-0.891 -3.765 -0.244 -0.138
2.376 -3.365 2.530 1.884
- 2.401 -27.012 2.534 1.746
w 4 -2.188 -4.187 -19.893 - 19.090 0.274 -0 0.201
-0.505 -0.883 -3.602 -3.704 - 0.252 -0.099 -0.205
1.046 1.680 - 2.498 - 3.087 3.051 2.307 3.149
- 1.647 -3.390 - 25.993 -25.881 3.073 2.207 3.146
electrostatic
total
-0.108 -0.009
0.967 -1.108 0.845 0.620
-9.546 -15.894 0.737 0.610
-1.506 -2.419 -2.506 -2.017 -0.103 -0.011 -0.138
0.521 0.304 -0.979 -0.731 1.179 0.877 1.210
-7.978 -11.955 -16.208 - 13.845 1.075 0.865 1.071
E m exchange
electrostatic
total
-1.998 -2.242 -0.107 -0.012
0.687 -0.689 0.615 0.392
-10.337 -13.820 0.408 0.380
-1.745 -2.557 - 2.284 -1.826 -0.112 -0.016 -0.137
0.518 0.255 -0.846 -0.581 0.989 0.758 1.010
-9.089 -12.684 - 14.296 -12.038 0.877 0.743 0.873
core-resonance
EA
S,N4
,
S
- 206.13 5
N
- 196.049 -210.174
s1,2
S,-S2 Sl-N, N,-N, N7-N, Sl-s, s3-s4
s3,4
-207.983
N
-196.511
S,-N, S4-N7 N7-N8
N7-N5 N7-N6
- 3.887
- 19.882
TABLE IV: Atomic ( E A ) and the Interatomic Energies ( E m ) in As4S, and &,Si
EA
core-resonance h 4 S 4
As
- 131.943
S
-220.416
As1-As2
As,-S,
ss-s,
s,-s7 4
.
2
As,, S
-133.206
- 133.516 -221.056
As,-As, AS,-As4 As,-S,
As,-Ss s5-s6
s,-s7 S,-S,
- 8.581 -12.380 -0 -0 As4S4-6.993 -9.840 - 12.724 -11.097 -0 -0 -0
(in eV)
EAB exchange -1.931 - 2.405
TABLE V: Atomic ( E A ) and the Interatomic Energies ( E A B )in As,Se4 and As4Se,- (in eV) EA
As
-132.677
Se
-191.863
As,,,
- 133.290
4
Se
4
-133.837 -192.819
As,-As, As,-Se, Se,-Se6 Se,-Se7 &,-As,
As,-As, As,-Se, As ,-Se Se5-Se, Se,-Se, Se,-Se,
core-resonance As4Se4 -9.026 -10.889 -0 -0 As4Se4-7.862 - 10.382 -11.167 -9.631 -0 -0 -0
TABLE VI: Vertical Electron Affinities of S4N, present study, we assume that the open-shell molecular and A s , X , ~(in eV) orbitals in the doublet or the triplet state are occupied with one or two more electrons of a spin, respectively. product SA4 As4S4 4Se4 Neutral Species. The atomic net charge of S4N4in monoanion 2.907 1.170 2.141 Table I1 is considerably large, namely, S+0~709-N-0~709. (2.454) (1.091) (2.082) Previous calculations by the SCF-XCX-SW~~ and the dianion 0.196 -2.440 -0.115 CNDO/S MO methods@have yielded values of f0.55 and (-0.257) (-2.519) (-0.173) *0.47, respectively. Judging from a reasonable agreement a Bracketed values indicate the results after the CI of the present value with the experimental result, treatment of the total energies of the neutral species (see N-0.6,8gthose calculated for As4X, (As+o.456-S-o.456 and text). As+0~382-Se-o*3sz) would also be reliable. It is to be noted that As atoms are positively charged in As4X4according A 0 character. It seems proper, however, to consider that to the following order of electronegativity: As < Se < S two lone pairs and two S-N bonds on N consist in an C N.26 The large negative charges on N atoms in S4N4 intermediate hybridization between sp2 and sp3 referring indicate the possibility of the existence of two lone pairs to the bond angle at N, 113'. This is also suggested from on N as has been pointed out by Gopinathan and rather small value of the 2s A 0 density on N, 1.567, which Whiteheadeb using the localized MO technique, though is reminiscent of a considerable participation of the 2s A 0 they have demonstrated that one of the lone pairs is of 2s in the hybridization. On the contrary, in As4X4,the values
2124
The Journal of Physical Chemistry, Vol. 82, No. 19, 1978
Tanaka et ai.
I
KEY
LUMO (-2889eV
HOMO
6-10.089eV)
Flgure 2. The directions of atomic shifts in C p vgeometries (X =
S
or Se).
of the 3s and the 4s A 0 densities on S and Se as well as As show less participation of these s AO’s in the hybridizations. Consequently, the bond angles at these atoms ought to be near 90’ like the angles among the p AO’s as shown in Figure 1B and 1C. These tendencies are characteristic of atoms having electrons with large principal quantum numbers.27 The highest occupied molecular orbitals (HOMO) and S4N4consists of 2p, AOs of four N atoms in most part and of a little S-S pa portion in Figure 3A. Although this pattern seems to be rather different from the result of the CNDO/S MO method,%these destabilizations of 2p, AOs of N in the present calculation should be regarded as a more significant consequence of the electron richness of N atoms. On the other hand, the HOMO’S of As4X4,in which As atoms are almost completely trivalent and X divalent as shown in Tables IV and V, show quite different features in Figures 3B and 3C, that is, As-As pa bonds in As4S4,and almost nonbonding 4p, AO’s of four Se atoms and of As-Se (pa + p). bonds in As4Sel. The As-As pa bonds are more stabilized as the (HO - 2) and the (HO 3) MO’s in As4Sel. Two degenerate LUMOs of S4N4are mainly of S-S pa* bonds having negative eigenvalues (-2.689 eV). The existence of such low-lying LUMO’s is brought about also by the situation in which N atoms having unusual divalencies strip four S atoms of almost three electrons. This suggests that S4N4will easily accept at least an excess electron to stabilize more than the neutral species. The divalencies of N atoms are definitely shown by the existence of the repulsive N-N bonds arising from the electrostatic term in Table 111. It is also seen that both S-S and S-N bonds come from the core-resonance terms. The levels of the LUMOs of h 4 X 4are slightly higher than that of S4N4,which means less electron deficient properties for these species. These tendencies seem to be reasonable since S and Se atoms are divalent and As atoms trivalent in As4X4as usual. It is to be noted that the patterns of the LUMO’s of As4& are of more As-As pii and less As-As pa* types, where ii denotes the n-type MO parallel to the four-S (or Se) plane. The difference in the patterns of the LUMOs among S4N4,h4S4,and As4Se4is due to the order of destabilization of pa* bonds (i.e., the order of stabilization of pa bonds, conversely) between S-S in S4N4and between As-As in As4Xl. This feature is also reflected in
(4942eV) LUMO
E 1;Y
HOMO
6-10242eV)
HOMO
(-10399eV)
(C )
Figure 3. The orbltal patterns and levels of the HOMO and the two degenerate LUMOs of S4N4(A), As4S4(B), and As4%, (C). In the key, X deslgnates S or Se in As4S4 or As4Se,, respectively.
the Em values of S-S bonds in S4N4and of As-As bonds in As4X4in Tables 111-V. The most contributive terms in these EAB’Scome from the core-resonance mainly consisting of the pa bonds between the concerned atoms. Anionic Species. It is apparent that the vertical electron affinities yielding the monoanions are all positive from Table VI, that is, each monoanion is more stable than the neutral species. The stabilization of S4N4-is remarkable (a. 2.9 eV). This tendency does not change, even adopting the total energy of S4N4after the consideration of the configuration interaction (CI) including one-electron excitations and pair excitations within all the ranges of (HO - 6)MO to (LU + 6) MO. Similarly, the total energies of As4S4and As4Se4after the CI treatment do not reach those of the monoanions. In the usual sense, it is not likely that the monoanionic species are more stable than the neutral
The Journal of Pbysical Chemistry, Vol. 82, No. 19, 1978 2125
Electronlc Structures of S4N4,As4S4, and As4Se4
TABLE VII:
T h e A 0 Components of Spin Densities of
S.N,- a n d Aa,X,-
S
Px PY
Pz
&a4 0.271 0.001 0.033
s,,,
S
Px PW -.,
Pz S
PY - _. PY PZ
-0.001 0.023 -0.003 -0.003 N 0.006 0.059 0.001 0.022
As, ,*
0.024 0.072 - 0.004 0.001 4
4
0,001 0.255 0 -0.001 S
As,,,
0.018 0.028 -0.003 0.006 -4%4
0 0.275 0 -0.001 Se
0.007
0.005
0
0.004 0
0 0.07 0
0.080
ones, much less preceding the achievement of geometrical optimization through Jahn-Teller distortion. It is clear, from EA analyses (Tables 111-V), that the increase in the atomic stabilization energies of S in S4N4-and of As in As4X4-,which is accompanied by the relaxations of the positive charges on these atoms as shown in Table 11, is most contributive to the stabilizations of the monoanions. The same but more temperate tendencies are also seen in N or X atoms in each anionic species. On the other hand, all of the S-N and N-N bonds in S4N; and the As-X and X-X in As4X4-are destabilized as in the usual case. The S-S and As-As bonds in S4N; and in As4X;, respectively, are split into two sets on account of CZugeometries of the anions employed here. The a-singly occupied molecular orbital (SOMO) of these anions are essentially like one of the LUMOs of the neutral species (LUMO(a)) in Figures 3A-C. Although the average value of two kinds of S-S bonds in S4N4-are almost unchanged from that in S4N4, those of As-As bonds in As4X4-surmount those in As4X4 for the benefit of the As-As pe nature in the a-SOMOs. Thus the unexpected stability of SIN; is caused by the considerable increase in the atomic stabilization energies of S atoms exceeding other bond destabilizations. These atomic stabilizations of S in S 4 N originate ~ in rather anomalously large atomic polarities in neutral S4N4as has been mentioned above. It is also noted that the degrees of bond destabilizations in S4N4-are fairly small. For As4X4-,the increase in the atomic stabilization energies of As is rather relaxed according to the less atomic polarities in the neutral species. In the dianions, however, the systems begin to destabilize as usual because the increase in bond destabilizations overcomes the atomic stabilization energies and, in As4X?-, As-As bond stabilization. The A 0 components contributing most to the spin densities of the monoanions (Table VII) have a parallel trend with those in the a-SOMOs and the increased A 0 densities in the monoanions, namely, S-S pa* and As-As pii types in S4N4-and As4X4-,respectively. Calculated isotropic hyperfine coupling constants of 14Nin S4N; and S4N42- using the atomic parameters of Pople and Beveridgels are 2.18 and 4.36 G, respectively. This value for S4N4- is in p o d agreement with the experimental result, 1.185 G.9 I t is to be noted that these coupling constants are rather small compared with those of 14Nin other inorganic radicals (-13 G),29which may be attributed to the small spin density at 2s A 0 on N as shown in Table VII. The orbital pattern of the LUMO's in Figure 3 will be useful for the estimation of the degree of Jahn-Teller
distortions in the monoanions. The pattern of either of the two LUMOs of S4N4suggests that the distortion will be rather prevented on account of the S1-Szand S3-S4po* nature, when an electron is captured into the LUMO. On the other hand, in As4X4-, the Asl-As2 bond will be lengthened, while the opposite As9-As4 bond shortened because of the nature of one of the LUMOs (e.g., LUMO(a) in this case). The geometric instability of As4S4against electron attachment has also been pointed out concerning the electron-trapping unit in As2S3glass.14 Therefore, by virtue of both energetic and geometric stabilities of S4N,, S4N4will be of use as a new class of electron-acceptors consisting of charge transfer salts or of electron-trapping center, if it is adequately doped, in solid state materials such as chalcogenide glass and so on.
Conclusion We have examined the electronic structures of S4N4, As4S4,and As4Se4,and their anionic species. It has been shown that S4N4has a considerably large vertical electron affinity for the benefit of the increase in the atomic stabilization energies of S atoms in particular. The anomalous stabilizationsof S atoms in S4N; originate from the abnormal atomic positions in S4N4in which all N atoms are forced to be divalent. For As4S4and As4Se4,the vertical electron affinities are also positive but less than that of S4N4. Although we have not performed here the geometric optimizations of the anionic species, the analyses of the LUM0's of each species predict the geometric stability of S4N4-compared with As4S4-and As4Se4-. This may indicate a possible feature of S4N4molecule as a new class of electron-acceptor or trapping center in solid state materials. The difference in the LUMOs of these three species is essentially due to the degree of the S-S and the As-As bond energies. Further experimental and theoretical investigations concerning the present topics would be desirable.
Acknowledgment. We are grateful to the Data Processing Center of Kyoto University for its generous permission to use the FACOM M190 computer. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education of Japan (No. 255315). References and Notes M. Gregory, J . Pharm., 21, 315 (1835); 22, 301 (1835). (a) A. G. MacDlarmid, C. M. Mikulskl, P. J. Russo, M. S. Saran, A. F. Garito, and A. J. Heeger, J. Cbem. Soc., Cbem. Commun., 476 (1975); (b) C. M. Mikulskl, P. J. Russo, M. S. Saran, A. G. MacDiarmkl, A. F. Garito, and A. J. Heeger, J. Am. Cbem. Soc., 97, 6358 (1975); (c) M. J. Cohen, A. F. Garito, A. J. Heeger, A. 0. MacDlarmld, C. M. Mikulski, M. S. Saran, and J. Klepplnger, /bid., 98, 3844 (1976). (a) V. V. Walatka, Jr., M. M. Labes, and J. H. Perlstein, Pbys. Rev. Lett., 31, 1139 (1973); (b) C. Hsu and M. M. Labes, J. Cbem. Pbys., 61, 4640 (1974). R. L. Greene, G. B. Street, and L. J. Suter, Pbys. Rev. Lett.,35, 1799 11975). B. D. Sharma and J. Donohue, Acta Crystallogr., 16, 891 (1963). (a) A. G. Turner and F. S. Mortimer, Inorg. Cbem., 5, 906 (1968); (b) M. S. Gopinatkan and M. A. Whkehead, Can. J. Cbem., 53, 1343 11974). K. Tanaka, T. Yamabe, A. Noda, K. Fukul and H. Kato, J . Pbys. Cbem., 82, 1453 (1978). (a) D. Chapman and T. C. Waddington, Trans. Faraday Soc., 58. 1679 (1962); (b) F. S. Braterman, J. Cbem. Soc. A , 2297 (1965); (c) J. B. Mason, bid., 1567 (1969); (d) R. Glelter, ibld., 3174 (1970); (e) P. Cassoux, J. F. Labarre, 0. Glemser, and W. Koch, J. Mol. Struct., 13, 405 (1972); (f) D. R. Salahub and R. P. Messrner, J . Cbem. Pbys., 64, 2039 (1976); (9) W. R. Salaneck, J. W-P. Lln, A. Paton, C. B. Duke, and G. P. Ceasar, Phys. Rev. 13,13, 4517 (1976). (a) D. Chapman and A. G. Massey, Trans. Faraday Soc., 58, 1291 (1962); (b) R. A. Melnzer, D. W. Pratt, and R. J. Myers, J. Am. Cbem. SOC., 91, 6623 (1969). H. J. EmelBus, Endeavour, 32, 76 (1973).
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An Ab Initio Molecular Fragment Investigation of the Inversion Barrier and Syn-Anti Isomerization Mechanisms of Formaldoxime W.
F. Hwang and H. A. Kuska”
Depatfment of Chemjstty, The Universjty of Akron, Akron, Ohio 44305 (Received November 9, 1977: Revised Manuscript Received July 11, 1978)
The ab initio molecular fragment method is used to calculate two possible intermediate states in the syn-anti isomerization of formaldoxime. The results suggest that the intermediate is stabilized by a hybridization change which results in the formation of a linear C-N-0 a system.
Introduction The syn-anti isomerization of formaldoxime has been the subject of a number of experimental and theoretical p a p e r ~ . l - ~From J ~ these studies it appears that the interconversion takes place by an inplane lateral shift r
Chart I: Resonance I n t e r a c t i o n in t h e Intermediate State of F o r m a l d o x i m e
1
Case I:no resonance i n t e r a c t i o n
rather than a 180° rotation around the C-N bond. The previous theorectical papers1J2 suggest that the intermediate undergoes a nitrogen hybridization change; however, the possibility of a hybridization change on the X group was not investigated. In this paper the ab initio molecular fragment method of Chri~toffersenl~ is used to study this possibility.
Procedure For formaldoxime, the initial state (syn form) was constructed the same way as trans-forma1do~ime.l~ Two possible intermediate states were constructed as described in Chart I along with the molecular fragment date summarized in Table I. The two possible intermediate states utilize different oxygen lone pair hybridizations. The complete optimized geometry calculated by Liotard12 is adopted as our input nuclear coordinates for the intermediate state of formaldoxime. Results and Discussion The calculated total energies and the energy components are compared with that of the initial state in Table 11. 0022-365417812082-2126$0 t .OO/O
resonance i n t e r a c t i o n Case 11: “resonance interaction” b e t w e e n t h e f i l l e d n - t y p e 2p, o r b i t a l of o x y g e n a n d t h e C = N ?T system
As shown in Table 111, we find two significant results. First, without a hybridization change and resultant resonance interaction (as in case I) the inversion barrier, 74.9 kcal/mol, is too high to be acceptable. Second, with the resonance interaction scheme (case 11)the absolute E-N potential increases profoundly and stabilizes the system. An examination of the individual molecular orbitals reveals that a case I predominately oxygen lone pair orbital with an energy of -0.3752 au becomes a more stable C-N-0 a bonding orbital in case I1 (the energy is -0.4036 au and the carbon, nitrogen, and oxygen pr atomic orbital coefficients are 0.16, 0.47, and 0.77, respectively). The original case I C-0 a orbital has an energy of -0.1244 au and coefficients of 0.69 and 0.35 for the carbon and nitrogen. In case I1 it is slightly stabilized, -0.1287 au, and has carbon, nitrogen, and oxygen pr coefficients of 0.70, 0.38, and -0.52. 0 I978 American Chemical Society
