Roger S. Grev

Concerning zero‐point vibrational energy corrections to electronic energies

For comparison with experimentally obtained thermochemical data, zero‐point vibrational energies (ZPVEs) are required to convert total electronic energies obtained from ab initio quantum mechanical studies into 0 K enthalpies. The currently accepted practice is to employ self‐consistent‐field (SCF) harmonic frequencies that have been scaled to reproduce experimentally observed fundamental frequencies. This procedure introduces systematic errors that result from a recognizable flaw in the method, namely that the correct ZPVE, G(0), is not one half the sum of the fundamental vibrational frequencies. Until better methods for accurately determining ZPVEs are presented, we recommend using different scaling factors for the determination of ZPVEs than those used to compare theoretically determined harmonic frequencies to observed fundamentals.

An energetically low‐lying silacyclopropyne isomer of SiC2

We have discovered a low‐lying cyclic isomer of Si–C–C which is best described as a three‐membered ring with a weak carbon–carbon triple bond. In these theoretical studies a double zeta plus polarization basis set was used initially. At the self‐consistent‐field (SCF) and two‐configuration (TC) SCF levels of theory the ring structure is a transition state leading to linear Si–C–C. A configuration interaction (CI) treatment at the SCF optimized cyclic geometry, however, show it to lie 1.1 kcal/mol below the linear structure. Extension of the basis set to include a second set of d functions on each atom gives a final prediction that the ring structure lies ∼5 kcal below linear SiCC.

The ground state of Si3, two near degenerate isomers

Abstract Two low-lying states of the Si 3 molecule are found which are very close in energy. One is a C 2 , closed-shell 1 A 1 structure analogous to the ground state of Si 2 C. This structure has two short SiSi bonds with r (SiSi) = 2.160 A and a central angle of 78.1°. The second structure is a D 3h triplet, 3 A′ 2 , and has a bond length of 2.263 A. Theoretical studies using basis sets of triple-zeta plus double polarization (TZ + 2P) quality in conjunction with configuration interaction (Cl) techniques at the double-zeta plus polarization (DZP) self-consistent-field (SCF) or two-configuration SCF (TCSCF) optimized geometries predict these two states are within 1 kcal/mole of each other. Although our methods suggest that the singlet is lower in energy, we are unable to make a definitive conclusion as to which is the true ground state of Si 3 .

Analytic energy second derivatives for general MCSCF wave functions

Expressions for the determination of analytic energy second derivatives for general MCSCF wave functions are presented. Equations for two distinct approaches: (1) direct differentiation of the energy expression and associated Lagrangian condition; and (2) power series expansion of the Hamiltonian and exponential‐i‐lambda transformation of the wave function, are developed. The problem of the nonzero nullity of the Hessian, and the resultant existence of redundant variables in the coupled perturbed multiconfiguration Hartree Fock (CPMCHF) equations, is discussed and a straightforward solution proposed. The viability of the methods presented in this paper are illustrated by a sample calculation on formaldehyde, using a double zeta (DZ) basis set and including 325 MCSCF configurations in the state space.

The remarkable monobridged structure of Si2H2

Inspired by the observation of a monobridged structure of Si2H2 by Cordonnier et al. via microwave spectroscopy (see the following paper), we have reinvestigated the Si2H2 singlet state potential energy surface using large basis sets and extensively correlated wave functions. Coupled‐cluster single, double, and (perturbative) triple excitation methods [CCSD(T)] in conjunction with a triple‐zeta 2df (TZ2df ) basis set on silicon and a triple zeta with two sets of polarization (TZ2P) basis set on hydrogen predict that the monobridged Si(H)SiH structure is indeed a minimum; in fact, Si(H)SiH is the second most stable Si2H2 isomer, as suggested by a recent theoretical study [B. T. Colegrove and H. F. Schaefer, J. Phys. Chem. 94, 5593 (1990)]. The predicted Si(H)SiH geometrical structure—which exhibits the shortest SiSi bond distance of any molecule characterized to date—and hence the rotational constants, as well as the quartic centrifugal distortion constants are in good agreement with the experimental data....

X̃ 1A1, ã 3B1, and à 1B1 electronic state of silylenes. Structures and vibrational frequencies of SiH2, and SiHF, and SiF2

Abstract The spectroscopic characteristics of the three lowest-lying electronic states of SiH 2 , SiHF, and SiF 2 have been predicted via a priori quantum-mechanical methods. Where experimental results are available, the theoretical predictions usually agree well. Specifically Raos identification of the 3 B 1 state of SiF 2 is confirmed here. Also Milligan and Jacoxs very tentative assignment of an absorption at 2032 cm −1 to ν 1 (SiH 2 ) is shown to be reasonable. Many spectroscopic features have been predicted for which there are no experimental observations at present.

Thermochemistry of CHn, SiHn (n=0–4), and the cations SiH+, SiH2+, and SiH3+: A converged quantum mechanical approach

We have determined 0 K heats of formation of CHn and SiHn (n=0–4) as well as the cations SiH+, SiH2+, and SiH3+ using large atomic natural orbital basis sets and coupled cluster methods including all single, double, and (perturbatively) triple excitations [CCSD(T)]. Core‐correlation effects on the bond dissociation energies have been explicitly evaluated. For the intermediate hydrides CHn and SiHn (n=1–3), heats of formation are determined from theoretical bond dissociation energies in two ways: using experimental heats of formation of the H and C (or Si) atoms; and using experimental heats of formation of the H atom and the parent hydrides CH4 (or SiH4). In principle, this procedure allows us to place rigorous upper and lower bounds on the heats of formation of the intermediate hydrides. Because our theoretically predicted atomization energies are already of high quality, estimation of remaining deficiencies in the one‐particle basis sets can be obtained from extrapolation of observed trends in atomizati...

Geometrical structures and vibrational frequencies of the energetically low‐lying isomers of SiC3

The ground state of the SiC3 molecule is found to be a closed‐shell cyclic C2v symmetry structure which can be described as a four‐membered ring with a transannular (cross ring) carbon–carbon bond, r(C–C)=1.469 A. Theoretical studies with a triple‐zeta plus double‐polarization function (TZ2P) basis set in conjunction with the configuration‐interaction technique at the TZ2P self‐consistent‐field optimized geometries predict this rhomboidal structure to be 4.1 kcal/mol more stable than the linear triplet Si–C–C–C isomer. A second closed‐shell rhomboidal C2v symmetry structure with carbon–silicon transannular bonding, r(Si–C)=1.880 A, was located and characterized as a local minimum lying 4.3 kcal/mol above the ground‐state rhomboidal structure at this level of theory. Higher‐level theoretical methods, including contributions from triple excitations, with larger basis sets will be required to obtain a more definitive set of relative energies.

Structure and bonding in the parent hydrides and multiply bonded silicon and germanium compounds : from MHn to R2M=M'R2 and RM≡M'R

Publisher Summary The chapter describes the structure and bonding in the parent hydrides and multiply bonded silicon and germanium compound. Experimental and theoretical studies of unsaturated silicon and germanium compounds have produced a wealth of new and unexpected phenomena, and more is sure to come. Foremost among those already recognized is the enhanced stability of divalent isomers in these systems, but we should include, as well, the trans-bent nature of double- and triple-bonded systems, the curious inversion of single- and double-bond dissociation energies, and the emergence of low lying stable or ground state bridged isomers. Many of these new phenomena are foreshadowed by the properties of the parent hydrides. The reversal of the patterns in the sequential bond dissociation energies of CH, compared to SiH, and GeH, already gives a clue to the lack of any special stability associated with sp2 hybridized bonding environments. Furthermore, the pyramidal nature of silyl and germyl radicals, and the large barriers to linearity in SiH, and certainly GeH, as well, presages the instability of the planar and linear bonding environment customarily seen in alkenes and alkynes, respectively.

6‐311G is not of valence triple‐zeta quality

We have discovered a weakness in the construction of the 6‐311G basis set. The second set of three contracted s‐functions is, in fact, not a valence orbital but is instead a 1s function. Thus in the s space this basis set is better represented as 63‐11. Coupled with an earlier observation that additional diffuse p functions are required to yield results comparable to those of the Huzinaga–Dunning (10s6p/5s3p) triple‐zeta basis set in certain cases, this suggests that greater caution should be exercised in the use of the 6‐311G basis set.