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Pnictinidenes are an increasingly relevant species in main group chemistry and generally exhibit proclivity for the triplet electronic ground state. However, the elusive singlet electronic states are often desired for chemical applications. We predict the singlet‐triplet energy differences (ΔEST=ESinglet−ETriplet) of simple group 15 and 16 substituted pnictinidenes (Pn−R; Pn=P, As, Sb, or Bi) with highly reliable focal‐point analyses targeting the CCSDTQ/CBS level of theory. The only cases we predict to have favorable singlet states are P−PH2 (−3.2 kcal mol−1) and P−NH2 (−0.2 kcal mol−1). ΔEST trends are discussed in light of the geometric predictions as well as qualitative natural bond order analysis to elucidate some of the important electronic structure features. Our work provides a rigorous benchmark for the ΔEST of fundamental Pn−R moieties and provides a firm foundation for the continued study of heavier pnictinidenes. [ABSTRACT FROM AUTHOR]

A new estimator for three-center two-particle Coulomb integrals is presented. Our estimator is exact for some classes of integrals and is much more efficient than the standard Schwartz counterpart due to the proper account of distance decay. Although it is not a rigorous upper bound, the maximum degree of underestimation can be controlled by two adjustable parameters. We also give numerical evidence of the excellent tightness of the estimator. The use of the estimator will lead to increased efficiency in reduced-scaling one- and many-body electronic structure theories. [ABSTRACT FROM AUTHOR]

The global minimum on the Si2CH2 electronic singlet potential energy surface has been theoretically predicted to be a peculiar hydrogen bridged (Si···H···Si) disilacyclopropenylidene structure (Si2CH2). An accurate quartic force field for Si2CH2 has been determined employing ab initio coupled-cluster theory with single and double excitations and a perturbative treatment for triple excitations [CCSD(T)], in combination with the correlation consistent core-valence quadruple zeta (cc-pCVQZ) basis set. The vibration-rotation coupling constants, equilibrium and zero-point vibration corrected rotational constants, centrifugal distortion constants, and harmonic and fundamental vibrational frequencies for six isotopologues of Si2CH2 are predicted using vibrational second-order perturbation theory (VPT2). The anharmonic corrections for the vibrational motions involving the H bridged bonds are found to be more than 5% with respect to the corresponding harmonic vibrational frequencies. In this light, an experimental detection and characterization of disilacyclopropenylidene (Si2CH2) is highly desired. [ABSTRACT FROM AUTHOR]

The singlet ground (X 1Σ+) and excited (1Σ-,1Δ) states of HCP and HPC have been systematically investigated using ab initio molecular electronic structure theory. For the ground state, geometries of the two linear stationary points have been optimized and physical properties have been predicted utilizing restricted self-consistent field theory, coupled cluster theory with single and double excitations (CCSD), CCSD with perturbative triple corrections [CCSD(T)], and CCSD with partial iterative triple excitations (CCSDT-3 and CC3). Physical properties computed for the global minimum (X 1Σ+HCP) include harmonic vibrational frequencies with the cc-pV5Z CCSD(T) method of ω1=3344 cm-1, ω2=689 cm-1, and ω3=1298 cm-1. Linear HPC, a stationary point of Hessian index 2, is predicted to lie 75.2 kcal mol-1 above the global minimum HCP. The dissociation energy D0[HCP(X 1Σ+)→H(2S)+CP(X 2Σ+)] of HCP is predicted to be 119.0 kcal mol-1, which is very close to the experimental lower limit of 119.1 kcal mol-1. Eight singlet excited states were examined and their physical properties were determined employing three equation-of-motion coupled cluster methods (EOM-CCSD, EOM-CCSDT-3, and EOM-CC3). Four stationary points were located on the lowest-lying excited state potential energy surface, 1Σ-→1A″, with excitation energies Te of 101.4 kcal mol-1(1A″ HCP), 104.6 kcal mol-1(1Σ- HCP), 122.3 kcal mol-1(1A″ HPC), and 171.6 kcal mol-1(1Σ- HPC) at the cc-pVQZ EOM-CCSDT-3 level of theory. The physical properties of the 1A″ state with a predicted bond angle of 129.5° compare well with the experimentally reported first singlet state (Ã 1A″). The excitation energy predicted for this excitation is T0=99.4 kcal mol-1(34 800 cm-1,4.31 eV), in essentially perfect agreement with the experimental value of T0=99.3 kcal mol-1(34 746 cm-1,4.308 eV). For the second lowest-lying excited singlet surface, 1Δ→1A′, four stationary points were found with Te values of 111.2 kcal mol-1 (21A′ HCP), 112.4 kcal mol-1 (1Δ HPC), 125.6 kcal mol-1(2 1A′ HCP), and 177.8 kcal mol-1(1Δ HPC). The predicted CP bond length and frequencies of the 2 1A′ state with a bond angle of 89.8° (1.707 Å, 666 and 979 cm-1) compare reasonably well with those for the experimentally reported C 1A′ state (1.69 Å, 615 and 969 cm-1). However, the excitation energy and bond angle do not agree well: theoretical values of 108.7 kcal mol-1 and 89.8° versus experimental values of 115.1 kcal mol-1 and 113°. [ABSTRACT FROM AUTHOR]

The existence or nonexistence of GaH5 has been widely discussed [N. M. Mitzel, Angew. Chem. Int. Ed. 42, 3856 (2003)]. Seven possible structures for gallium pentahydride have been systematically investigated using ab initio electronic structure theory. Structures and vibrational frequencies have been determined employing self-consistent field, coupled cluster including all single and double excitations (CCSD), and CCSD with perturbative triples levels of theory, with at least three correlation-consistent polarized-valence-(cc-pVXZ and aug-cc-pVXZ) type basis sets. The X 1A′ state for GaH5 is predicted to be weakly bound complex 1 between gallane and molecular hydrogen, with Cs symmetry. The dissociation energy corresponding to GaH5→GaH3+H2 is predicted to be De=2.05 kcal mol-1. The H–H stretching fundamental is predicted to be v=4060 cm-1, compared to the tentatively assigned experimental feature of Wang and Andrews [J. Phys. Chem. A 107, 11371 (2003)] at 4087 cm-1. A second Cs structure 2 with nearly equal energy is predicted to be a transition state, corresponding to a 90° rotation of the H2 bond. Thus the rotation of the hydrogen molecule is essentially free. However, hydrogen scrambling through the C2v structure 3 seems unlikely, as the activation barrier for scrambling is at least 30 kcal mol-1 higher in energy than that for the dissociation of GaH5 to GaH3 and H2. Two additional structures consisting of GaH3 with a dihydrogen bond perpendicular to gallane (C3v structure 4) and an in-plane dihydrogen bond [Cs(III) structure 5] were also examined. A C3v symmetry second-order saddle point has nearly the same energy as the GaH3+H2 dissociation limit, while the Cs(III) structure 5 is a transition structure to the C3v structure. The C4v structure 6 and the D3h structure 7 are much higher in energy than GaH3+H2 by 88 and 103 kcal mol-1, respectively. [ABSTRACT FROM AUTHOR]

The singlet electronic ground state isomers, X 1Σg+ (AlOAl D∞h) and X 1Σ+ (AlAlO C∞ν), of dialuminum monoxide have been systematically investigated using ab initio electronic structure theory. The equilibrium structures and physical properties for the two molecules have been predicted employing self-consistent field (SCF) configuration interaction with single and double excitations (CISD), multireference CISD (MRCISD), coupled cluster with single and double excitations (CCSD), CCSD with perturbative triples [CCSD(T)], CCSD with iterative partial triple excitations (CCSDT-3 and CC3), and full triples (CCSDT) coupled cluster methods. Four correlation consistent polarized valence (cc-pVXZ) type basis sets were used. The AlAlO system is rather challenging theoretically. The two isomers are confirmed to have linear structures at all levels of theory. The symmetric isomer AlOAl is predicted to lie 81.9 kcal mol-1 below the asymmetric isomer AlAlO at the cc-pV(Q+d)Z CCSD(T) level of theory. The predicted harmonic vibrational frequencies for the X 1Σg+ AlOAl molecule, ω1=517 cm-1, ω2=95 cm-1, and ω3=1014 cm-1, are in good agreement with experimental values. The harmonic vibrational frequencies for the X 1Σ+ AlAlO structure, ω1=1042 cm-1, ω2=73 cm-1, and ω3=253 cm-1, presently have no experimental values with which to be compared. With the same methods the barrier heights for the isomerization AlOAl→AlAlO and AlAlO→AlOAl reactions were predicted to be 84.3 and 2.4 kcal mol-1, respectively. The dissociation energies D0 for AlOAl (X 1Σg+) and AlAlO (X 1Σ+)→AlO (X 2Σ+)+Al (2P) were determined to be 130.8 and 48.9 kcal mol-1, respectively. Thus, both symmetric AlOAl (X 1Σg+) and asymmetric AlAlO (X 1Σ+) isomers are expected to be thermodynamically stable with respect to the dissociation into AlO (X 2Σ+) + Al (2P) and kinetically stable for the isomerization reaction (AlAlO→AlOAl) at sufficiently low temperatures. [ABSTRACT FROM AUTHOR]

The two lowest-lying (H-bridged, cyclic) electronic states ([sup 2]B[sub 1] and [sup 2]A[sub 1]) of the disilaethynyl (SiSiH) radical have been investigated using ab initio electronic structure theory. Theoretical methods through the full coupled cluster with all triple excitations (CCSDT) have been used, and basis sets as large as Dunning’s correlation consistent pentuple set adopted. While the SCF, MP2, CISD, and CCSD levels of theory predict the [sup 2]B[sub 1] state to be lower in energy, the CCSD(T) and CCSDT methods show that the [sup 2]A[sub 1] state is the true electronic ground state. With our most reliable method, the energy difference is predicted to be T[sub e]([sup 2]B[sub 1])=0.60 kcal/mol (0.026 eV,210 cm[sup -1]) and T[sub 0]([sup 2]B[sub 1])=0.37 kcal/mol (0.016 eV,128 cm[sup -1]). This theoretical finding confirms the experimental assignment by Xu et al. [J. Chem. Phys. 108, 7645 (1998)] in 1998 that the ground state of SiSiH is the [sup 2]A[sub 1] state and it is 0.020±0.005 eV lower in energy than the [sup 2]B[sub 1] state. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The reactivity trends for two basic organic reactions – addition of bromine to a CC double bond in alkenes and electrophilic aromatic benzylation of alkyl‐ and halobenzenes – are rationalized using both experimental and theoretical quantities. Literature kinetic data are employed. The shifts of phenol O–H stretching frequency upon π‐hydrogen bonding (Δν(OH)exp) with the nucleophilic reactants are shown to provide a quantitative measure of the reactivity for the considered reactions. Very good correlations between Δν(OH)exp and theoretically estimated charge density parameters (atomic charges and shifts of molecular electrostatic potential) show that the O–H frequency shifts reflect the variations of nucleophilic properties in the reaction centers upon structural changes. The Δν(OH)exp shifts perform much better in quantifying reactivity trends than usually employed experimental quantities, such as gas‐phase basicities and proton affinities. [ABSTRACT FROM AUTHOR]

The 2A′ ground state of GeOH–HGeO system has been investigated by ab initio electronic structure theory. The equilibrium geometries and physical properties including dipole moments, harmonic vibrational frequencies, and associated infrared (ir) intensities for GeOH, HGeO, and the isomerization (1,2 hydrogen shift) transition state are determined at the self-consistent-field (SCF) and configuration interaction with single and double excitations (CISD) levels of theory with four basis sets. There appear to be two minima for the bent HGeO (isomers A and B) on its SCF and CISD potential energy hypersurfaces. At the Hartree–Fock level the structure with HGeO angle near 90° (isomer B) lies lower, but correlated methods show that the structure with HGeO angle near 120° (isomer A) actually lies lower. At the optimized CISD geometries, the single point energies of coupled cluster with single and double excitations (CCSD) and CCSD with perturbative triple excitations [CCSD(T)] methods are also determined. In the correlated procedures three different types of frozen core orbital approximation (15 frozen core, 10 frozen core, and 6 frozen core orbitals) have been examined. The energetics based on the first (15 frozen core orbitals) approximation present errors of about 1 kcal/mol compared to more accurate second (10 frozen core orbitals) and third (6 frozen core orbitals) approximations. At the highest level of theory employed in this research, CCSD(T) with triple zeta plus double polarization with diffuse and higher angular momentum functions [TZ2P(f,d)+diff] basis set, the bent GeOH molecule is predicted to be lower in energy than the bent HGeO molecule by 28.5 kcal/mol. This energy separation becomes 25.7 kcal/mol with the zero-point vibrational energy (ZPVE) correction. The classical energy barrier for the exothermic isomerization reaction [HGeO(B)→GeOH] is determined to be 11.8 kcal/mol and the activation energy (with the ZPVE... [ABSTRACT FROM AUTHOR]

Ab initio electronic structure theory has been employed in order to investigate the ground state potential energy hypersurface of the SiOH+–HSiO+ system. Geometries and physical properties including dipole moments, harmonic vibrational frequencies, and infrared intensities of two equilibrium and isomerization (1,2 hydrogen shift) reaction transition state were determined. The self-consistent-field, configuration interaction with single and double excitations, coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory were used with five basis sets. At the highest level of theory employed in this study, CCSD(T) using the triple zeta plus double polarization with diffuse and higher angular momentum functions basis set, linear SiOH+ is predicted to be more stable than linear HSiO+ by 66.1 kcal/mol. This energy difference becomes 64.1 kcal/mol with an appropriate zero-point vibrational energy (ZPVE) correction. At the same level of theory, the classical barrier height for the exothermic isomerization (1,2 hydrogen shift) reaction HSiO+→SiOH+ is determined to be 29.3 kcal/mol and the activation energy (with the ZPVE correction) is 27.3 kcal/mol. The geometrical and energetic features are compared with those of the valence isoelectronic HBO–BOH, HCO+–COH+, and AlOH–HAlO systems. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ab initio electronic structure theory has been used to characterize the lowest 1A‘ excited state of isocyanogen. The geometries, vibrational frequencies, and infrared intensities of the X 1Σ+ and A 1A‘ states have been determined using the single and double excitation configuration-interaction (CISD) method in conjunction with a triple-ζ plus double polarization (TZ2P) basis set. Adding the Davidson correction and including a set of f polarization functions in the basis set (CISD+Q TZ2Pf), we estimate the energy difference between the ground and first excited singlet state of isocyanogen to be Te = 41 700 cm-1. With a C–N–C bond angle of 117°, this bent 1A‘ excited state may be useful in stimulated emission pumping (SEP) experiments on the NCCN[larger_closed_square]CNCN isomerization. [ABSTRACT FROM AUTHOR]

The ground and excited electronic states of the s-tetrazine molecule have been studied using the methods of ab initio electronic structure theory. In particular, complete self-consistent field (SCF) optimizations of the equilibrium structures on the X 1Ag, a 3B3u, and A 1Au(C2h)/1B3u (D2h) surfaces using both double-ζ (DZ) and DZ+polarization (DZ+P) basis sets have been carried out. Harmonic vibrational frequencies have been analytically evaluated at these stationary points. DZ SCF results for higher excited electronic states are also reported with the optimizations on these surfaces having been restricted to D2h symmetry. Single point configuration interaction energies including single and double excitations relative to the SCF references (CISD) have been used to predict both vertical and adiabatic electronic excitation energies for all states investigated herein. In addition the Davidson correction [CISD(+Q)] and the closed shell coupled cluster singles and doubles (CCSD) method have been used to approximate the effect of higher excitations. Our results for the equilibrium geometries, harmonic vibrational frequencies, electronic excitation energies, and the energies of the photodissociation of s-tetrazine are analyzed and compared to spectroscopic results where available. [ABSTRACT FROM AUTHOR]

Investigates systematically the two triplet ([sup 3]A[sub 2] and [sup 3]B[sub 2]) and two singlet ([sup 1]A[sub 2] and [sup 1]B[sub 2]) excited states of CH[sub 2] using ab initio electronic structure theory. Electronic structure considerations; Geometries; Dipole moments; Harmonic vibrational frequencies and infrared intensities; Energetics.

Ab initio molecular electronic structure theory has been applied to the D6h- symmetry isomer of Si6H6. Self-consistent-field (SCF) theory has been applied in conjunction with double zeta (DZ) and double zeta plus silicon 3d (DZ+d) basis sets. The DZ+d basis set may be designated Si(11s 7p 1d/6s 4p 1d), H (4s/2s). The structure of hexasilabenzene is predicted to be: re(Si–Si) =2.223 Å, re(Si–H) =1.476 Å (DZ SCF); re(Si–Si) =2.205 Å, re(Si–H) =1.463 Å (DZ+d SCF). Vibrational frequencies have been predicted at the DZ SCF level, at which the DZ SCF structure is a true minimum. However, the lowest vibrational frequency is predicted to be only 10 cm-1. [ABSTRACT FROM AUTHOR]

Electronegative substituents such as F favor closed-shell singlet, while electropositive substituents such as Li favor triplet ground electronic states in carbenes and silylenes. Therefore, SiLiF presents a very instructive case study of the competing electronic effects involved. We have examined the minima on the lowest-lying 1A’, 1A‘, and 3A‘ potential surfaces of SiLiF, using self-consistent-field and configuration interaction methods with a basis set at the double zeta plus polarization level. Two minima, one with a wide and the other with a narrow bond angle θ (LiSiF), have been found for each of the three electronic states, in analogy with SiHLi. The most stable minimum at all levels of approximation is the narrow-angle triplet (3A‘ I). It lies below the second most stable minimum, the narrow-angle closed-shell singlet (1A’ I), by 10.5 kcal/mol at the CISD level of theory. This result is similar to the singlet–triplet separation in SiHLi at the same level of theory (10.7 kcal/mol), demonstrating that the electropositive Li atom prevails against the electronegative F. The harmonic vibrational frequencies and the results of a Mulliken population analysis are also presented for each minimum. [ABSTRACT FROM AUTHOR]

The potential energy hypersurface of the SiOH–HSiO system has been investigated using ab initio electronic structure theory. The geometries and physical properties including dipole moments, harmonic vibrational frequencies, and associated infrared (IR) intensities for the two equilibrium and isomerization (1,2 hydrogen shift) transition state structures have been determined employing self-consistent-field (SCF) and configuration interaction with single and double excitations (CISD) methods. At the CISD optimized geometries, single point energies of the three stationary points were evaluated using coupled cluster with single and double excitations (CCSD) and CCSD with perturbative triple excitations [CCSD(T)] levels of theory. In the correlated procedures three different frozen core schemes (6 frozen core, 2 frozen core, and 0 frozen core) have been applied to examine the importance of 1s, 2s, and 2p core electrons. With the SCF method two isomers (A and B) were found for HSiO. However, at the CISD level of theory structure B with the bond angle of about 93° has collapsed to structure A with the bond angle of about 122°, confirming the findings of lower level studies. At the highest level of theory, CCSD(T) with triple zeta plus double polarization (TZ2P) augmented with higher angular momentum and diffuse functions TZ2P(f,d)+diff basis set, TZ2P(f,d)+diff CCSD(T), the energy separation between SiOH and HSiO is predicted to be 12.1 kcal/mol. This energy separation becomes 9.8 kcal/mol with the zero-point vibrational energy (ZPVE) correction. With the same method the classical energy barrier for the exothermic isomerization reaction (HSiO→SiOH) was determined to be 25.8 kcal/mol and the activation energy (with the ZPVE correction) becomes 24.1 kcal/mol. The two frozen core approximations have generated 0.005 Å (6 frozen core) and 0.001 Å (2 frozen core) in error for the SiO bond length compared to no frozen core method. In... [ABSTRACT FROM AUTHOR]

Ab initio electronic structure theory has been employed in order to investigate the ground state potential energy hypersurfaces of the HBO–BOH system. Geometries, dipole moments, harmonic vibrational frequencies, and infrared intensities of two equilibrium and two transition state [inversion (bending through linear geometry) and isomerization] structures were determined at the self-consistent-field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory using three basis sets. The theoretically predicted geometries and physical properties agree very well with available experimental values. At the highest level of theory employed in this study, CCSD(T) using triple zeta plus double polarization with higher angular momentum function [TZ2P(f,d)] basis set, the linear HBO molecule is predicted to be 45.0 kcal/mol more stable relative to the bent BOH species; with the zero-point vibrational energy (ZPVE) correction this energy separation becomes 44.4 kcal/mol; the classical barrier height for the inversion motion of the bent BOH molecule is predicted to be 3.5 kcal/mol and the barrier height with the ZPVE correction is 3.0 kcal/mol; the classical activation energy for the isomerization (1,2 hydrogen shift) reaction BOH→HBO is determined to be 29.4 kcal/mol and the activation energy with the ZPVE correction is 26.6 kcal/mol. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The ground state potential energy hypersurface of the HCO+–COH+ system has been reinvestigated systematically using high level ab initio electronic structure theory. The geometries and physical properties of the two equilibrium and one isomerization transition state structures were determined at the self-consistent-field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory with nine basis sets. First, the optimized geometries of the three stationary points at twenty eight (28) levels of theory were discussed. Second, the characteristics and responses of the molecular orbitals (MOs) with respect to the normal coordinates have been elucidated via energy derivative analysis technique. Third, dipole moments, harmonic vibrational frequencies, and infrared (IR) intensities were described. Finally, the relative energies among the stationary points have been compared. At the highest level of theory, CCSD(T) using triple zeta plus double polarization with diffuse and higher angular momentum functions [TZ2P(f,d)+diff] basis set, the linear HCO+ molecule is found to be 39.7 kcal/mol [37.7 kcal/mol with zero-point vibrational energy (ZPVE) correction] lower in energy relative to the linear COH+ molecule. At the same level of theory the activation energy for the isomerization reaction HCO+→COH+ is predicted to be 76.9 kcal/mol (72.6 kcal/mol with the ZPVE correction). Once the COH+ species is formed, therefore, it should be reasonably stable with respect to the isomerization reaction at low temperatures. [ABSTRACT FROM AUTHOR]

Ab initio molecular electronic structure methods have been used to examine the rearrangement of methylnitrene (CH3N) to methyleneimine (CH2NH) on the lowest-lying singlet surface. Geometries have been optimized and harmonic vibrational frequencies obtained for both methylnitrene and methyleneimine as well as for the transition states connecting them. Basis sets up to triple zeta plus double polarization plus f functions (TZ2P+f ) in size were used. Equilibrium geometries have been determined at the self-consistent field (SCF) and the single and double excitation configuration interaction (CISD) levels of theory. Because the 1E state of CH3N exhibits a Jahn–Teller splitting, the resulting 1A’ and 1A‘ states have been examined in order to determine their respective roles on the potential energy surface. After identifying and analyzing a larger 1A’–1A‘ splitting artifact due to symmetry breaking, the true classical Jahn–Teller splitting is predicted to be <0.01 kcal mol-1. This theoretical investigation is the first to consider the effects of electron correlation in conjunction with extended basis sets on the energetics of the methylnitrene 1,2-hydrogen shift. It is found that the conclusion of most previous lower level studies—that there is no barrier to rearrangement on the 1A’ surface—appears to be justified. If Cs symmetry is enforced, there is a sizable barrier (∼30 kcal mol-1) on the 1A‘ potential surface. [ABSTRACT FROM AUTHOR]

There exists a discrepancy between theory and experiment for the N2S bending vibrational frequency. Ab initio molecular electronic structure theory has been used in order to investigate the electronic ground state of the dinitrogen sulfide (N2S) molecule. Self-consistent field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD) wave functions were employed using the three basis sets, the double zeta plus polarization (DZP), triple zeta plus double polarization (TZ2P), and TZ2P with one set of higher angular momentum polarization functions (TZ2Pf). For CCSD wave functions the connected triple excitations were included through perturbation theory [CCSD(T)]. Using the CCSD(T) method with the larger basis sets, experimental physical properties including bond lengths, rotational constant, centrifugal distortion constant, l-type doubling constant, vibrational frequencies, and isotopic shifts for the N–N stretching frequency have been reproduced with quantitative accuracy. This analysis proves the essential correctness of an earlier theoretical prediction of the N2S harmonic bending vibrational frequency. [ABSTRACT FROM AUTHOR]

An investigation concerning nine electronically low-lying doublet states of the SNS molecule appeared recently. The present paper represents the conclusion of studies on the NS2 molecule, focusing on the low-lying SNS quartet states. Nine quartet states, including six bent structures (4A2, 4B2, two 4A1, and two 4B1 ) and three linear structures (4Πg and two 4Πu ), have been investigated at the self-consistent field (SCF), single and double excitation configuration interaction (CISD), and complete active space (CAS) SCF levels of theory with five basis sets, double zeta (DZ) through triple zeta plus double polarization (TZ+2P). Four of these quartet states lie within 1.8 eV (42.0 kcal mol-1 ) of the 2A1 ground state of NS2. Thus one or more of these quartet states (which lie much lower than for the valence isoelectronic NO2 molecule) should be observable. At the SCF level of theory the stability of the wave functions and the relative energies of the nine quartet states are discussed in conjunction with the molecular orbital (MO) Hessian. [ABSTRACT FROM AUTHOR]

Ab initio molecular electronic structure methods have been used, in conjunction with large basis sets, to investigate the possible existence of isomers of the conventional C2v open-chain NO2 and its anion, NO-2. Equilibrium geometries have been optimized and harmonic vibrational frequencies obtained for both the ‘‘peroxy’’ and cyclic isomers of these species at the self-consistent-field (SCF), single and double excitation configuration interaction (CISD), coupled cluster including all single and double substitutions (CCSD), and complete active space (CAS) SCF levels of theory utilizing both double-zeta plus polarization (DZ+P) and quadruple-zeta plus double polarization (QZ+2P) basis sets. For comparison, ‘‘normal’’ (C2v open-chain) NO-2 was also studied. Diffuse functions were added to these basis sets to provide a better description of the electron densities of the anionic isomers. The four peroxy and cyclic structures investigated were predicted to be relatively high-lying (≥60 kcal mol-1) minima at most levels of theory considered. Moreover, interconversion between the C2v and Cs open-chain forms appears unlikely both for NO2 and NO-2. [ABSTRACT FROM AUTHOR]

The importance of including triple and quadruple excitations (relative to a single Hartree–Fock determinant) in ab initio electronic structure configuration interaction (CI) theory is investigated for several small molecules [HF, N2, CO, H2O, NH3, (3B1) CH2, and (1A1) CH2]. Specifically the effects of these high order electron correlations on equilibrium molecular geometries, dipole moments, harmonic vibrational frequencies, and infrared intensities are reported. Triple and quadruple excitations are generally found to affect the dipole moment, in an absolute sense, only slightly. In some cases, infrared intensities show a medium to large dependence on higher excitations. Molecular geometries, and subsequently the harmonic vibrational frequencies, however, are significantly more dependent upon these higher excitations. Quadruple excitations are found to be significantly more important than triple excitations for all closed shell systems except for CO, where the relative importance of triples to quadruples is roughly 2:3 in predicting for vibrational frequencies. On the other hand, it is found that triples and quadruples are of nearly equal importance for 3B1 CH2. The equilibrium bond length and molecular properties of the multiply bonded species CO and N2 show a larger absolute dependence on the higher than double excitations. Several additional levels of theory [e.g., all singles, doubles, and quadruples (CISDQ)] have been applied to HF and N2 in a more detailed investigation of the structure of the CI Hamiltonian matrix. It is concluded that only a very small subset of the triply and quadruply excited configurations account for nearly all of the higher excitation effects on harmonic frequencies. [ABSTRACT FROM AUTHOR]

Ab initio molecular electronic structure theory has been used in an attempt to characterize the low-lying stationary points on the H+5 potential energy hypersurface. Three distinct levels of theory have been used: the self-consistent-field (SCF) method, configuration interaction (CI) including all single and double excitations, and full configuration interaction. Four different basis sets were used: double zeta (DZ), double zeta plus polarization (DZP), an extended basis set designated H (6s2p/4s2p), and a second extended basis set designated H (8s3p/6s3p). The higher levels of theory are in agreement that the only minimum for H+5 is a C2v structure, with three other stationary points (of D2d, C2v, and D2h symmetries) lying less than 1 kcal/mol higher in energy. The predicted dissociation energy D0 is 5.5 kcal/mol, which is estimated to be about 1 kcal/mol less than the exact D0. Furthermore, there are six other stationary points lying less than 8 kcal/mol above the minimum. Vibrational frequencies, dipole moments, and infrared intensities for each of the ten stationary points have been predicted at several different levels of theory. From the perspective of quantum chemistry, the H+5 system is very attractive as a candidate for the study of the vibrational dynamics of weakly bound systems. [ABSTRACT FROM AUTHOR]

Molecular beam experiments (lithium atom plus hydrogen fluoride) by both Becker and co-workers (C.H. Becker, P. Casavecchia, P.W. Tiedemann, J.J.Valentini, and Y.T. Lee, J. Chem. Phys.73, 2833 (1980)) and Loesch and Stienkemeier (H.J. Loesch and F. Stienkemeier, J. Chem. Phys.98, 9570 (1993)) deduced a van der Waals complex of type Li···HF. In this research, molecular electronic structure theory [aug-cc-pCVQZ CCSD(T)] has been used to predict a well depth of 0.86 kcal mol−1relative to separated Li + HF. However, the barrier from this vdW well to the more strongly bound (∼6.2 kcal mol−1) HFLi complex lies 0.43 kcal mol−1belowseparated Li + HF. [ABSTRACT FROM AUTHOR]

Although evanescent at best, the bare dimers of the elements Cr, Mo, and W have been identified as possible candidates for the sextuple metal–metal bond. The corresponding dibenzene sandwich compounds Cr2(C6H6)2, Mo2(C6H6)2, and W2(C6H6)2, satisfy the ‘18-Electron Rule’, and might achieve high-order metal–metal bonds and longer lifetimes at the same time. Twenty-two different DFT methods have been used to evaluate this possibility. Based on the present Wiberg bond index and molecular orbital analyses, however, only quadruple metal–metal bonds are predicted for the electronic ground states of Cr2(C6H6)2, Mo2(C6H6)2, and W2(C6H6)2, rather than the sextuple or even quintuple bonds, for both singlet and triplet electronic states. It is possible to force the hypothesisedD6hsextuple bond electron configuration, but the resulting energy is 39 kcal/mol above theD2hquadruple bond structure for Cr2(C6H6)2. However, the constrained sextuple bond structure for Mo2(C6H6)2lies 19 kcal/mol above the MoMo singlet. For W2(C6H6)2the sextuple bond structure is predicted to lie only 3 kcal/mol above the WW structure. Thus the answer to the question raised in our title is ‘almost’ for the tungsten–sextuple bond. [ABSTRACT FROM PUBLISHER]

The optimized geometries, total energies, and vibrational frequencies of the polyacenes and perfluoropolyacenes in their closed-shell singlet and lowest triplet states have been studied using the B3LYP method in conjunction with double-ζ plus polarization (DZP) basis sets. The equilibrium structures of all the polyacenes and perfluoropolyacenes are planar, with D2h symmetry, and the singlet and triplet geometries display interesting patterns for each family of molecules. The largest singlet-triplet structural changes appear in the outermost C-C distances. The small perfluoropolyacene systems have ground state singlet states, and the singlet-triplet energy gaps decrease as the number of linearly fused benzene rings increases. With the number of rings greater than seven, the triplet state of the perfluorinated species falls below the closed-shell singlet. For the parent polyacenes, the same shift occurs between n = 8 and n = 9. Correspondingly, the LUMO-HOMO gap for the singlet state decreases with increasing number of linearly fused perfluoro benzene rings. The predicted singlet-triplet separations for the perfluorinated compounds range from 54 kcal mol-1 (perfluoronaphthalene) to -7 kcal mol-1 (n = 10). [ABSTRACT FROM AUTHOR]

The equilibrium structure of the ammonium radical is determined by an extrapolation procedure of large [up to CCSD(T)/aug-cc-pV5Z] coupled cluster calculations and analysis of experimental rotational constants corrected for effects of rotation-vibration interaction. These approaches yield r[sub e] values of 1.0367 and 1.0363 Å, respectively. The small difference serves as a valuable internal consistency check and suggests that r[sub e] in NH[sub 4] lies in the range 1.0365±0.0005 Å. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The two lowest-lying (X˜ [sup 2]A[sup ″] and A˜ [sup 2]A[sup ′]) electronic states and lowest linear stationary point (1 [sup 2]Π) of the 1-silaketenyl radical (HSiCO) have been investigated systematically using ab initio electronic structure theory. The lowest linear stationary point possesses two distinct imaginary vibrational frequencies along the HSiC bending coordinates, indicating a strong Renner-Teller interaction. The ground and first excited states of HSiCO are found to have trans-planar bent structures and they are more distorted from linearity but less polar than the corresponding states of HCCO. Specifically, the X˜ [sup 2]A[sup ″] structure features a small HSiC bond angle of 84°. With our most reliable method, cc-pVQZ CCSD(T), the classical X˜-A˜ splitting has been predicted to be 35.7 kcal/mol (1.55 eV, 12 500 cm[sup -1]). The barriers to linearity were determined to be 53.5 kcal/mol (2.32 eV, 18 700 cm-1) for the X˜ [sup 2]A[sup ″] state and 17.8 kcal/mol (0.77 eV, 6240 cm-1) for the A˜ [sup 2]A[sup ′] state. The ground state of HSiCO was found to be relatively stable thermodynamically against the two dissociation reactions HSiCO(X˜ [sup 2]A[sup ″])→H([sup 2]S)+SiCO(X˜ [sup 3]Σ[sup -]) and HSiCO(X˜ [sup 2]A[sup ″])→SiH(X˜ [sup 2]Π)+CO(X˜ [sup 1]Σ[sup +]). Due to the large infrared (IR) intensities of some of the vibrational modes, IR spectroscopic investigation of the HSiCO radical may be feasible. HSiCO is the global minimum for these four atoms, lying energetically below SiCOH (38.5 kcal/mol), HCSiO (40.7 kcal/mol), and CSiOH (76.3 kcal/mol) at the TZ2P(f,d) configuration interaction with single and double excitations (CISD) level of theory. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: The lowest energy structure for Mn2(BF)(CO)10 is predicted to consist of two Mn(CO)5 groups bridged by a BF group without a manganese–manganese bond. This structure is related to the stable compounds Mn2(μ-BX)(CO)10 (X=Cl, Br) and [(η5-C5H5)Ru(CO)2]2(μ-BF), which have been synthesized and characterized by X-ray crystallography. The following principles determine the lowest energy structures of the Mn2(BF)(CO) n (n =9, 8, 7, 6) derivatives: (1) two-electron donor bridging μ-BF groups are highly favorable; (2) four-electron donor bridging η2-μ-BF groups are not found and thus appear to be highly unfavorable. Thus the lowest energy structure of Mn2(BF)(CO)9 is a doubly bridged structure with bridging CO and BF groups, in contrast to the experimentally observed unbridged structure of Mn2(CO)10. The lowest energy structures of Mn2(BF)(CO)8 have either a four-electron donor η2-μ-CO group or a two-electron donor bridging BF group. Similarly the lowest energy structures of the more highly unsaturated Mn2(BF)(CO) n (n =7, 6) are singlet (for n =7) or triplet (for n =6) states in which the BF group is a bridging rather than a terminal ligand. [ABSTRACT FROM AUTHOR]