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There is considerable interest, from both experimental and theoretical perspectives, in molecules incorporating multiple bonds between main group elements. Herein, we not only consider the parent molecules HE=EH (E=As, Sb, Bi), but also a number of their isomers. For each E2H2 molecule, a number of different structures were optimized with four different DFT methods. Final structures were determined with the coupled cluster method CCSD(T) using large basis sets, namely cc‐pVQZ‐PP, incorporating relativistic psuedopotentials. All feasible dissociation pathways are examined. For all three E2H2 molecules the trans isomer lies lowest in energy, with the cis isomer higher by 2.7 (As), 2.1 (Sb), and 1.8 (Bi) kcal mol−1, respectively. However, both cis and trans structures should be observable, as large barriers (27.7, 20.5, and 17.7 kcal mol−1) separate them. For both the cis and trans structures, in the infrared the strong E‐H stretching frequencies should also be observable. Only the cis structures have dipole moments (0.62, 0.01, and 0.83 debye, respectively), and their observation by microwave spectroscopy would be stunning. Also considered were the higher energy vinylidene‐like, pyramidal, monobridged, and linear structures. We conclude that molecules such as HSb=SbH‐Fe(CO)4, HBi=BiH‐Fe(CO)4, and related systems, should be feasible synthetic targets. [ABSTRACT FROM AUTHOR]

Symmetric and asymmetric triple excitation corrections for the orbital-optimized coupled-cluster doubles (OO-CCD or simply 'OD' for short) method are investigated. The conventional symmetric and asymmetric perturbative triples corrections [(T) and (T)Λ] are implemented, the latter one for the first time. Additionally, two new triples corrections, denoted as OD(Λ) and OD(Λ)T, are introduced. We applied the new methods to potential energy surfaces of the BH, HF, C2, N2, and CH4 molecules, and compare the errors in total energies, with respect to full configuration interaction, with those from the standard coupled-cluster singles and doubles (CCSD), with perturbative triples [CCSD(T)], and asymmetric triples correction (CCSD(T)Λ) methods. The CCSD(T) method fails badly at stretched geometries, the corresponding nonparallelity error is 7-281 kcal mol-1, although it gives reliable results near equilibrium geometries. The new symmetric triples correction, CCSD(Λ), noticeably improves upon CCSD(T) (by 4-14 kcal mol-1) for BH, HF, and CH4; however, its performance is worse than CCSD(T) (by 1.6-4.2 kcal mol-1) for C2 and N2. The asymmetric triples corrections, CCSD(T)Λ and CCSD(Λ)T, perform remarkably better than CCSD(T) (by 5-18 kcal mol-1) for the BH, HF, and CH4 molecules, while for C2 and N2 their results are similar to those of CCSD(T). Although the performance of CCSD and OD is similar, the situation is significantly different in the case of triples corrections, especially at stretched geometries. The OD(T) method improves upon CCSD(T) by 1-279 kcal mol-1. The new symmetric triples correction, OD(Λ), enhances the OD(T) results (by 0.01-2.0 kcal mol-1) for BH, HF, and CH4; however, its performance is worse than OD(T) (by 1.9-2.3 kcal mol-1) for C2 and N2. The asymmetric triples corrections, OD(T)Λ and OD(Λ)T, perform better than OD(T) (by 2.0-6.2 kcal mol-1). The latter method is slightly better for the BH, HF, and CH4 molecules. However, for C2 and N2 the new results are similar to those of OD(T). For the BH, HF, and CH4 molecules, OD(Λ)T provides the best potential energy curves among the considered methods, while for C2 and N2 the OD(T) method prevails. Hence, for single-bond breaking the OD(Λ)T method appears to be superior, whereas for multiple-bond breaking the OD(T) method is better. [ABSTRACT FROM AUTHOR]

The unique importance of the cyclopropenylidene molecule conveys significance to its low-lying isomers. Eleven low-lying electronic triplet stationary points as well as the two lowest singlet structures of C3H2 have been systematically investigated. This research used coupled cluster (CC) methods with single and double excitations and perturbative triple excitations [CCSD(T)] and Dunning’s correlation-consistent polarized valence cc-pVXZ (where X=D, T, and Q) basis sets. Geometries, dipole moments, vibrational frequencies, and associated infrared intensities of the targeted molecules have been predicted. The electronic ground state of cyclopropenylidene (3S, the global minimum) is the X 1A1 state with C2v point group symmetry. Among the 11 triplet stationary points, 7 structures are found to be minima, 2 structures to be transition states, and 2 structures to be higher-order saddle points. For the six lowest-lying triplet structures and singlet propadienylidene (2S), relative energies (zero-point vibrational energy corrected values in parentheses) with respect to the global minimum [ X 1A1 (3S)] at the cc-pVQZ-UCCSD(T) level of theory are predicted to be propynylidene 3B(1aT)15.5(11.3)≤propadienylidene[1A1(2S)]14.2(13.3)≤propadienylidene[3B1(2T)]45.0(43.8)≤cyclopropenylidene[3A(3aT)]53.8(52.5)≤cyclopropyne[3B2(4aT)]70.0(70.6)≤cyclopropenylidene [3B1(3cT)]71.2(69.9)≤trans-propenediylidene [3A″(5T)]73.6(70.7) kcal mol-1. The combined energy for the [C(3P)+C2H2(X 1Σg+)] system with respect to the global minimum (3S) is determined to be 107.4(103.8) kcal mol-1. Therefore, these six triplet equilibrium structures are all energetically well below the dissociation limit to [C(3P)+C2H2(X 1Σg+)]. [ABSTRACT FROM AUTHOR]

Microhydration effects on cytosine and its radical anion have been investigated theoretically, by explicitly considering various structures of cytosine complexes with up to five water molecules. Each successive water molecule (through n=5) is bound by 7–10 kcal mol-1 to the relevant cytosine complex. The hydration energies are uniformly higher for the analogous anion systems. While the predicted vertical detachment energy (VDE) of the isolated cytosine is only 0.48 eV, it is predicted to increase to 1.27 eV for the lowest-lying pentahydrate of cytosine. The adiabatic electron affinity (AEA) of cytosine was also found to increase from 0.03 to 0.61 eV for the pentahydrate, implying that the cytosine anion, while questionable in the gas phase, is bound in aqueous solution. Both the VDE and AEA values for cytosine are smaller than those of uracil and thymine for a given hydration number. These results are in qualitative agreement with available experimental results from photodetachment-photoelectron spectroscopy studies of Schiedt et al. [Chem. Phys. 239, 511 (1998)]. [ABSTRACT FROM AUTHOR]

The amino group in the nucleic acid bases frequently interacts with other bases or with other molecular systems. Thus any nonplanarity of the amino group may affect the molecular recognition of nucleic acids. Ab initio Hartree-Fock (HF) and second-order Mo\ller-Plesset perturbation (MP2) levels of theory have been used to obtain the equilibrium geometries of the Cl and Cs structures for five common nucleic acid bases. The energy barriers between the Cl and Cs structures have also been predicted. A series of correlation consistent basis sets up to cc-pCVQZ and aug-cc-pVQZ has been used to systematically study the dependence of the amino group nonplanarity. The equilibrium geometries of the nucleic acid bases with an amino group, including adenine, guanine, and cytosine, are examined carefully. At the MP2 level of theory, larger basis sets decrease the extent of nonplanarity of the amino group, but the decrease slows down when the QZ basis sets are used, demonstrating the intrinsic property of nonplanarity for guanine. For adenine and cytosine the situation is less clear; as the HF limit is approached, these two structures become planar. Addition of core correlation effects or diffuse functions further decreases the degree of nucleic acid base nonplanarity, in comparison to the original cc-pVXZ (X=D, T, and Q) basis sets. The aug-cc-pVXZ basis shows smaller degrees of nonplanarity than the cc-pCVXZ sets. The aug-cc-pVXZ basis is less size dependent than the cc-pVXZ and cc-pCVXZ sets in the prediction of the amino-group-related bond angles and dihedral angles and energy barriers for adenine, guanine, and cytosine. The cc-pCVQZ and aug-cc-pVQZ MP2 results may be regarded as benchmark predictions for the five common bases. The predicted classical barriers to planarization are 0.02 (adenine), 0.74 (guanine), and 0.03(cytosine) kcal mol-1. [ABSTRACT FROM AUTHOR]

An analysis of molecular properties is presented for several first-row diatomic molecules as determined at the CCSD(T) level of theory. In particular, a comparison of spectroscopic constants predicted by the ROHF-CCSD(T) methods of Scuseria vs Gauss et al. is given. Accurate determination of the values of equilibrium bond lengths, dissociation energies, harmonic vibrational frequencies, anharmonic constants, vibration–rotation coupling constants, and centrifugal distortion constants show only small differences between the two methods. In particular, we show that the average absolute differences between the two methods for the equilibrium bond length, harmonic vibrational frequency, and the dissociation energy are 0.000 09 Å, 0.8 cm-1 and 0.1 kcal/mol, respectively. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ab initio quantum mechanical self-consistent-field (SCF) and single and double excitation configuration interaction (CISD) methods have been used in a new study of hydrogen bonding between the water molecule and the hydroxyl radical. The basis sets used are double-zeta plus polarization (DZP) and triple-zeta plus double polarization (TZ2P). The two energetically low-lying minima are 2A’ and 2A‘ states, with hydrogen bonding occurring between the oxygen atom in the water molecule and the hydrogen atom in the hydroxyl radical; the 2A’ state has a slightly (∼0.3 kcal/mol) lower energy. The comparable planar C2v symmetry structure lies <0.1 kcal/mol higher in energy. The hydrogen bond distance H...O of the 2A’ is ∼1.94 Å, which is close to that of the water dimer. The ground state dissociation energy is 5.6 kcal/mol, which is larger than that of the water dimer. The predicted infrared spectra for these structures are also reported. [ABSTRACT FROM AUTHOR]

Molecular structures, harmonic vibrational frequencies, and infrared intensities for diborane B2H6, dialane Al2H6, and digallane Ga2H6 have been determined using high level ab initio quantum mechanical methods. The highest level of theory employed in this study is the single and double excitation coupled cluster (CCSD) method with double-zeta plus polarization (DZP) basis sets. This study serves to assess the reliabilities of various theoretical methods, laying the foundation for future theoretical studies of larger boron, aluminum, and gallium molecular systems. It appears that the theoretical methods employed here are adequately reliable when compared with existing experiments. Several shortcomings of previous ab initio quantum mechanical studies on diborane(6) and its analogs have been addressed. [ABSTRACT FROM AUTHOR]

The importance of multiple sets of d functions on the energy of SO2 and other molecules of Si, P, S, and Cl has been tested by comparing total energies from the SWPS set (supplementation by five sets of d functions as recommended by Strömberg, Wahlgren, Pettersson, and Siegbahn) with those obtained with one or two sets of d functions. The energy improvement due to supplementation is very sensitive to the nature of the attached atoms and to valence shell expansion but the optimum d function exponents are not appreciably affected by either of these factors. When used in conjunction with Dunning–Hay basis sets for second row atoms, supplementation by a single set of d functions provides about 80% of the self-consistent field (SCF) energy lowering obtained with five sets; use of a second set generates about 15% more. These results are compared with the results of adding f or replacing nuclear-centered functions with bond functions. Varying d function exponents about the optimum values produces an energy surface with the same general shape for all ten molecules studied. Based on this uniformity within a limited sample of molecules containing second row elements, recommendations are presented for the inclusion of supplementary sets of d functions in basis sets and the level of energy improvement to be achieved by using them is estimated. [ABSTRACT FROM AUTHOR]

Analytic gradient methods have been used to predict the harmonic vibrational frequencies and the infrared (IR) intensities of HCN, HNC, CO2, CH4, NH4+, C2H2, H2O, H2CO, and NH3 at the single, double, and perturbative triple excitation coupled-cluster [CCSD(T)] level of theory. All studies were performed using a double zeta plus polarization (DZ+P) basis set with a contraction scheme of (9s5p1d/4s2p1d) for C, N, and O and (4s1p/2s1p) for H. The results of previous studies using the same basis set with self-consistent field (SCF), single and double excitation configuration interaction (CISD), and the single and double excitation coupled-cluster (CCSD) methods are included to allow a detailed comparison. The theoretical harmonic frequencies of all nine molecules are compared to the 28 (out of a total of 35 possible) known experimental harmonic vibrational frequencies. IR intensities are also compared to known experimental values. The absolute average error in frequencies for the CCSD(T) method with respect to experiment was 2.4%. Values of 9.1%, 3.7%, and 2.3% have been reported previously for the SCF, CISD, and CCSD methods, respectively. The CCSD(T) method is the only one for which a significant number of the theoretical vibrational frequencies lie below experiment. If the absolute errors for two frequencies that are known to be described inadequately with the DZP basis set are not included in the averages, they become 8.6% (SCF), 3.6% (CISD), 1.7% (CCSD), and 1.5% [CCSD(T)]. The establishment of an average error for the DZP CCSD(T) method should improve the reliability in the prediction of unknown experimental frequencies. [ABSTRACT FROM AUTHOR]

The titanium tetrahydride molecule has been investigated using several levels of ab initio molecular electronic structure theory in conjunction with a basis set of size Ti(14s11p6d/10s8p3d), H(5s1p/3s1p). The self-consistent field, single and double excitation configuration interaction, and single and double excitation coupled cluster methods were used to predict the molecular structure, harmonic vibrational frequencies, infrared and Raman intensities. Results for all five isotopic species TiH4, TiH3D, TiH2D2, TiHD3, and TiD4 are presented. A careful comparison with the recent matrix isolation infrared results of Xiao, Hauge, and Margrave is made where experimental data exist. All four of the tentatively assigned experimental vibrational features are given strong support from theory. [ABSTRACT FROM AUTHOR]

The present study aims to characterize the electronically low-lying doublet states of the SNS molecule in detail. From an analysis of the eigenvalues and eigenvectors of the molecular orbital (MO) Hessian at the self-consistent-field (SCF) level, the corresponding rotations amongst the high-lying occupied and low-lying unoccupied molecular orbitals highlight nine low-lying doublet states worthy of study. Three bent (2A1, 2B2, and 2A2), three ring (2B1, 2A2, and 2A1), and three linear (2Πu and two 2Πg) structural isomers with doublet electronic states have been investigated at the SCF, configuration interaction with single and double excitations (CISD), and complete active space (CAS) SCF levels of theory with five different basis sets, double zeta (DZ) through triple zeta plus double polarization (TZ+2P). At the SCF level of theory, the stability of the wave functions and the energetics of the above mentioned doublet states are discussed in terms of the molecular orbital (MO) Hessian. The MO Hessian is often shown to properly predict the energetic ordering of the different electronic states. [ABSTRACT FROM AUTHOR]

The full coupled cluster singles, doubles, and triples (CCSDT) model for molecular electronic structure theory has been used in conjunction with two less complete coupled cluster methods (CCSD, CCSDT-1) to predict the equilibrium bond distances and harmonic vibrational frequencies of a family of closed-shell ground state diatomic molecules. The results are compared with the configuration interaction method including all single, double, triple, and quadruple excitations (CISDTQ) and experiment. With previous CCSDT results for HF, OH-, N2, and CO, the present work provides a realistic assessment of the CCSDT method. The BeO molecule in particular places great demands on the theory, and the linearized triple excitation model CCSDT-1 fails badly. [ABSTRACT FROM AUTHOR]

We have completed theoretical studies on the 14-valence–electron molecules CCO, CNN, SiCO, and SiNN in their 3Σ- linear ground states. The theoretical methods that we employed were ab initio self-consistent field (SCF), configuration interaction including single and double excitations from the dominant configuration (CISD), and complete active space (CASSCF). The extent of bonding of the C or Si atom with the CO or N2 ligand is significantly different in each case. The strongest bond is formed between the C and N atoms in CNN and the weakest between the Si and N atoms in SiNN. Only for CNN does the resultant geometry vary significantly with the theoretical method. Our theoretical harmonic frequencies are compared to the experimental infrared spectra reported for these molecules in matrix isolation studies. The theoretical and experimental results are easily brought into harmony for CCO and SiCO, but not for CNN and SiNN. [ABSTRACT FROM AUTHOR]

A single and double excitation coupled cluster (CCSD) method restricted to closed-shell single configuration reference functions is described in explicit detail. Some significant simplifications resulting from the restriction to closed-shell systems are exploited to achieve maximum computational efficiency. Comparisons for energetic results and computational requirements are made with the single and double excitation configuration interaction (CISD) method. The specific molecules considered include N2, H2O, H3O+, H5O+2, HSOH, and s-tetrazine (C2N4H2). [ABSTRACT FROM AUTHOR]

The general restricted Hartree–Fock (RHF) closed-shell energy analytic third derivative method is presented together with details concerning its implementation and that of the derivative integral algorithm. The viability of the energy equation and the integral techniques are illustrated by calculations on water, hydrogen cyanide, and formaldehyde with double zeta (DZ), double zeta plus polarization (DZP), triple zeta (TZ), triple zeta plus polarization (TZP), and triple zeta plus two sets of polarization (TZ2P) basis sets. Cubic force constants in reduced normal coordinates are reported for these basis sets. The cubic force constants are then applied in the prediction of the vibration–rotation interaction constants αBr (B=A, B, C) for these molecules. [ABSTRACT FROM AUTHOR]

The harmonic vibrational frequencies and infrared intensities of the H2Si=CH2 molecule have been predicted by ab initio molecular quantum mechanics. Self-consistent-field (SCF) and configuration interaction (CI) methods have been used in conjunction with a double zeta plus polarization (DZ+P) basis. Comparison with pertinent experimental data is made. [ABSTRACT FROM AUTHOR]

The theory for the ab initio evaluation of potential energy gradients at the configuration interaction level of accuracy is presented, with special attention to the treatment of the various types of orbitals (frozen core, active, frozen virtual) which may arise. The new method has been used for a study of the D3h and C2v geometries of S3. SCF and CISD calculations predict the D3h structure to be lower than the C2v by 9.3 and 2.3 kcal/mol, respectively, whereas CASSCF and MRCISD predict the C2v structure to be the lowest by 8.9 and 8.2 kcal/mol, respectively, using good basis sets. These calculations support the prediction of Carlsen and Schaefer that both forms should be observable experimentally. [ABSTRACT FROM AUTHOR]

The prediction of the diagonal correction to the Born–Oppenheimer approximation is now possible by ab initio analytic methods, as has recently been shown by Yarkony and Lengsfield. At the general restricted Hartree–Fock (GRHF) level of approximation, the procedure is straightforward: solutions of the coupled perturbed Hartree–Fock equations (CPHF) and some overlap integrals are all that are required. This correction is evaluated for a series of small molecules with various basis sets: H2O, H2O+, CH2, HCF, H+5, and F2. It is interesting to observe that the value of this correction (0.11 kcal) for the singlet–triplet splitting of CH2 is larger than the relativistic correction, and that the theoretical value for Tnre (BO)≡9.23±0.20 kcal has come even closer to the best ab initio prediction of 9.4 kcal. [ABSTRACT FROM AUTHOR]

An MCSCF procedure of the complete active space (CAS) genre has been used to predict the structures and energetics of the HCCN molecule. Full configuration interaction (CI) within the π-electron space (1πx, 1πy, 2πx, 2πy, 3πx, 3πy orbitals, six electrons) was used in these MCSCF studies. With double zeta plus polarization (DZP) and triple zeta plus double polarization (TZ2P) basis sets, the bent structure lies 6.2 and 5.7 kcal, respectively, below the linear conformation. Multireference CI wave functions reduce this energy difference significantly. [ABSTRACT FROM AUTHOR]

Twenty seven new isomers, considerably lower in energy than the two previously reported structures of the novel molecule periodane, LiBeBCNOF, are found theoretically using a stochastic search methodology at the B3LYP/6–31G(d) level of theory. The global minimum is predicted to lie 122 kcal/mol lower in energy than the previously claimed lowest energy structure. The importance of an automated isomer search method to help find isomers on the potential energy surface of molecules in the gas phase is emphasized. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [ABSTRACT FROM AUTHOR]

Ab initio prediction of the adiabatic electron affinity of CF have been carried out at the self-consistent-field, configuration interaction, and coupled-cluster levels of theory. The most accurate result is 0.40 eV, from the single, double, and the perturbatively connected triple excitation coupled cluster method with Dunning’s augmented correlation consistent polarized valence quadruple ζ basis set. This result disagrees with the earlier experimental report (3.2±0.4 eV), but agrees qualitatively with one of the published density functional results (0.53 eV). © 1994 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An analytic method for the evaluation of Ramanintensities from closed-shell self-consistent-field wave functions is presented. Predictioinsf or ethylenemolecule are also reported. (AIP) [ABSTRACT FROM AUTHOR]

Abstract: Hypercoordinate boron is most unusual, leading to considerable theoretical and experimental research on the parent BH5 molecule. The deprotonation energies of BH5 and the related molecules AlH5 and GaH5 have been of particular interest. Here the energy differences for are computed to be 332.4 and 326.3kcalmol−1, respectively, with an aug-cc-pVQZ basis set at the CCSD(T) level of theory. Vibrational frequencies for and are also reported as 1098, 1210, 2263, and 2284cm−1 and 760, 779, 1658, and 1745cm−1, respectively, again at the CCSD(T) aug-cc-pVQZ level of theory. Comparisons with the valence isoelectronic GaH5 molecule are made. [Copyright &y& Elsevier]

Five different pure density functional theory (DFT) and hybrid Hartree-Fock/DFT methods have been used to search for the molecular structures, thermochemistry, and electron affinities of the bromine hydrogen fluorides HBrFn/HBrFn- (n = 2, 4). The basis sets used in this work are of double-ζ plus polarization quality in conjunction with s- and p-type diffuse functions, labeled as DZP++. Structures with Br-F and Br-H normal bonds, that is, HBrF2/HBrF2- with C2v or Cs symmetry and HBrF4/HBrF4- with C4v or Cs symmetry, are genuine minima. However, unlike the original BrF3 and BrF5 molecules, the global minima for HBrFn/HBrFn- (n = 2, 4) species are predicted to be complexes, some of which contain hydrogen bonds. The demise of the hypervalent structures is due to the availability of favorable dissociation products involving HF, which has a much larger dissociation energy than F2. Similar reasoning suggests that PF4H, SF3H, SF5H, ClF2H, ClF4H, AsF4H, SeF3H, and SeF5H will all be hydrogen bond structures incorporating diatomic HF. The most reasonable theoretical values of the adiabatic electron affinities (EAad) are 3.69 (HBrF2) and 4.38 eV (HBrF4) with the BHLYP method. These electron affinities are comparable to those of the analogous molecules: Br2Fn, ClBrFn, and BrFn+1 systems. The first F-atom dissociation energies for the neutral global minima are 60 (HBrF2) and 49 kcal/mol (HBrF4) with the B3LYP method. The first H-atom dissociation energies for the same systems are 109 (HBrF2) and 116 kcal/mol (HBrF4). The large Br-H bond energies are not sufficient to render the hypervalent structures energetically tenable. The dissociation energies for the complexes to their fragments are relatively small. [ABSTRACT FROM AUTHOR]