Zero-point Vibrational Energy Corrections Research Articles

Intrinsic conformational preferences of and an anomeric‐like effect in 1‐substituted silacyclohexanes

AbstractA series of ab initio computations have been performed to estimate the CCSD(T) complete basis set limit of the axial/equatorial energy difference for the chair conformation of 1‐fluoro, 1‐chloro, 1‐methyl, 1‐hydroxy, and 1‐methoxysilacyclohexanes. The equatorial conformation is more stable than the axial by 0.21 kcal mol−1 for 1‐methylsilacyclohexane, while the axial position is more stable for 1‐fluoro, 1‐chloro, and 1‐methoxysilacyclohexane (by 0.09, 0.40, and 0.15 kcal mol−1, respectively). The axial and equatorial orientations of 1‐hydroxysilacyclohexane are nearly electronically isoenergetic (equatorial favored by 0.03 kcal mol−1). Zero point vibrational energy corrections have very little effect on the relative energies (less than 0.08 kcal mol−1). These results suggest the presence of an interesting stereoelectronic effect, similar to the anomeric effect, in these silacyclohexanes even without an additional heteroatom, like O, in the ring. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Coupled-cluster studies of the lowest excited states of the 11-cis-retinal chromophore

The first few excited states of the 11-cis-retinal (PSB11) chromophore have been studied at the coupled-cluster approximative singles and doubles (CC2) level using triple-zeta quality basis sets augmented with double sets of polarisation functions. The two lowest vertical excitation energies of 2.14 and 3.21 eV are in good agreement with recently reported experimental values of 2.03 and 3.18 eV obtained in molecular beam measurements. Calculations at the time-dependent density functional theory (TDDFT) level using the B3LYP hybrid functional yield vertical excitation energies of 2.34 and 3.10 eV for the two lowest states. Zero-point vibrational energy (ZPVE) corrections of -0.09 and -0.17 eV were deduced from the harmonic vibrational frequencies for the ground and excited states calculated at the density functional theory (DFT) and TDDFT level, respectively, using the B3LYP hybrid functional.

Structure and spectral features of H+(H2O)7: Eigen versus Zundel forms

The two dimensional (2D) to three dimensional (3D) transition for the protonated water cluster has been controversial, in particular, for H(+)(H(2)O)(7). For H(+)(H(2)O)(7) the 3D structure is predicted to be lower in energy than the 2D structure at most levels of theory without zero-point energy (ZPE) correction. On the other hand, with ZPE correction it is predicted to be either 2D or 3D depending on the calculational levels. Although the ZPE correction favors the 3D structure at the level of coupled cluster theory with singles, doubles, and perturbative triples excitations [CCSD(T)] using the aug-cc-pVDZ basis set, the result based on the anharmonic zero-point vibrational energy correction favors the 2D structure. Therefore, the authors investigated the energies based on the complete basis set limit scheme (which we devised in an unbiased way) at the resolution of the identity approximation Moller-Plesset second order perturbation theory and CCSD(T) levels, and found that the 2D structure has the lowest energy for H(+)(H(2)O)(7) [though nearly isoenergetic to the 3D structure for D(+)(D(2)O)(7)]. This structure has the Zundel-type configuration, but it shows the quantum probabilistic distribution including some of the Eigen-type configuration. The vibrational spectra of MP2/aug-cc-pVDZ calculations and Car-Parrinello molecular dynamics simulations, taking into account the thermal and dynamic effects, show that the 2D Zundel-type form is in good agreement with experiments.

Elementary constituents of microdevices: The Ge2H fragment

Highly correlated ab initio electronic structure theory has been used to systematically investigate the linear (X 2Pi) GeGeH and H-bridged (X 2B1 and A 2A1) GeHGe structures and the isomerization transition state (A 2A') connecting X 2Pi with A 2A1. The equilibrium structures and physical properties have been predicted employing 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)]. Four correlation-consistent polarized valence-[cc-pVXZ and aug-cc-pVXZ (X=T and Q)] type basis sets were used in the study. For the H-bridged GeHGe isomer, the X 2B1 ground state is predicted to lie only 0.74 [0.95 with zero-point vibrational energy (ZPVE) corrections] kcal mol-1 below the A 2A1 excited state at the CCSD(T) level of theory with the augmented correlation-consistent polarized valence quadruple zeta (aug-cc-pVQZ) basis set. The X 2B1 state of the H-bridged GeHGe isomer lies 8.6 kcal mol-1 below the X 2Pi ground state of the linear GeGeH isomer. The forward isomerization barrier from the linear ground state to the A 2A' transition state is predicted to be 3.7 kcal mol-1. The reverse isomerization barrier between the A 2A1 GeHGe structure and the X 2Pi GeGeH structure is predicted to be 11.6 (10.8 with the ZPVE corrections) kcal mol-1 at the aug-cc-pVQZ CCSD(T) level of theory.

Adiabatic approximations to internal rotation

A number of subtle and confusing issues are addressed concerning large amplitude motion (LAM) coordinates (chi) for internal molecular motions, using the methyl rotation in acetaldehyde (CH(3)CHO) as a model problem. If the LAM coordinate is chosen to be one of the H-C-C-O dihedral angles rho(1), rho(2), or rho(3), it lacks the required 2pi3 periodicity, and its use is thus undesirable. An excellent local internal coordinate for this model problem is tau(3)=13(rho(1)+rho(2)+rho(3)-2pi). A similarly good but nonlocal coordinate for the adiabatic approximation of internal rotation is provided by the intrinsic reaction coordinate s. Comparison of the mass-independent V(0)(tau(3)) and the mass-dependent V(0)(s) internal rotation curves shows that the two are virtually identical for the parent isotopolog of acetaldehyde. A unified internal coordinate projection scheme for determining complementary vibrational frequencies and subsequently V(ZPVE)(chi) along a path for LAM has been formulated, where V(ZPVE)(chi) is the zero-point vibrational energy correction to the internal rotation curve. In addition to its simplicity, the projection scheme developed for a distinguished reaction path generated by constrained optimizations is appealing because the vibrational frequencies along the LAM path are invariant to chemically meaningful choices of the internal coordinates for the complementary modes.

Isomerization of C3H3NO isomers: ab initio study

The structures and isomerization process of C3H3NO species have been explored at the MP2/6–311++G(d,p) level of theory of the ab initio method. Eleven minima and four interconversion transition states have been identified. The zero-point vibrational energy corrections were made to predict reliable energies. We predict a five-membered ring-like structure to be the lowest energy isomer, which is 177.73 kcal mol−1 more stable than the least stable isomer X found on the potential energy surface. The transition states and minima isomers were verified by frequency calculation. Intrinsic reaction coordinate (IRC) calculations have been performed to confirm that each transition state is linked by the desired reactants and products. The isomer stabilities have been studied using the relative energies, chemical hardness and chemical potential. The MHP principle could not predict the order of stability for C3H3NO isomers as arrived at with the relative energies. The role of intramolecular hydrogen bonds on the equilibrium structure has been discussed. The energy barrier and reaction enthalpy have been calculated during isomerization.

Accurateab initioPredictions of Ionization Energies and Heats of Formation for Cyclopropenylidene, Propargylene and Propadienylidene

The ionization energies (IEs) of cyclopropenylidene (c-C3H2), propargylene (HCCCH) and propadienylidene (H2CCC) have been computed using the CCSD(T)/CBS method, which involves the approximation to the complete basis set (CBS) limit at the coupled cluster level with single and double excitations plus quasi-perturbative triple excitation effect (CCSD(T)). The zero-point vibrational energy correction, the core-valence electronic correction, the scalar relativistic effect and the high level correction beyond the CCSD(T) excitations have also been made in these calculations. The CCSD(T)/CBS values for the IE(c-C3H2) and IE(HCCCH) of 9.164, 8.987 eV are in good agreement with the experimental values of (9.15 0.03) and (8.96 0.04) eV. The CCSD(T)/CBS calculations yield the IE values of 10.477 and 10.388 eV for the ionization transitions H2CCC H2CCC+(2A1, C2v) and H2CCC H2CCC+(2A0, Cs), respectively. On the basis of the FranckCondon factor consideration, the IE of (10.43 0.02) eV determined in the previous single-photon ionization experiment most likely corresponds to the ionization threshold for the H2CCC H2CCC+(2A1, C2v) transition. Although the precision of the experimental IE measurements for c-C3H2, HCCCH, and H2CCC is insufficient to pin down the accuracy of the theoretical calculations to better than 30 meV, the excellent agreement between the experimental and theoretical IE values observed in the present study indicates that the CCSD(T)/CBS calculations together with high-order correlation corrections are capable of yielding reliable IE predictions for simple hydrocarbon carbenes and bi-radicals. We have also reported the heats of formation at 0 K (Hof0) and 298 K (Hof298) for c-C3H2/c-C3H2+, HCCCH/HCCCH+, and H2CCC/H2CCC+. The available experimental Hf0o and Hof298 values for c-C3H2/c-C3H2+, HCCCH/HCCCH+ are found to be in good accord with the CCSD(T)/CBS predictions after taking into account the experimental uncertainties.

Accurate ab initio predictions of ionization energies and heats of formation for the 2-propyl, phenyl, and benzyl radicals

The ionization energies (IEs) for the 2-propyl (2-C(3)H(7)), phenyl (C(6)H(5)), and benzyl (C(6)H(5)CH(2)) radicals have been calculated by the wave-function-based ab initio CCSD(T)/CBS approach, which involves the approximation to the complete basis set (CBS) limit at the coupled cluster level with single and double excitations plus quasiperturbative triple excitation [CCSD(T)]. The zero-point vibrational energy correction, the core-valence electronic correction, and the scalar relativistic effect correction have been also made in these calculations. Although a precise IE value for the 2-C(3)H(7) radical has not been directly determined before due to the poor Franck-Condon factor for the photoionization transition at the ionization threshold, the experimental value deduced indirectly using other known energetic data is found to be in good accord with the present CCSD(T)/CBS prediction. The comparison between the predicted value through the focal-point analysis and the highly precise experimental value for the IE(C(6)H(5)CH(2)) determined in the previous pulsed field ionization photoelectron (PFI-PE) study shows that the CCSD(T)/CBS method is capable of providing an accurate IE prediction for C(6)H(5)CH(2), achieving an error limit of 35 meV. The benchmarking of the CCSD(T)/CBS IE(C(6)H(5)CH(2)) prediction suggests that the CCSD(T)/CBS IE(C(6)H(5)) prediction obtained here has a similar accuracy of 35 meV. Taking into account this error limit for the CCSD(T)/CBS prediction and the experimental uncertainty, the CCSD(T)/CBS IE(C(6)H(5)) value is also consistent with the IE(C(6)H(5)) reported in the previous HeI photoelectron measurement. Furthermore, the present study provides support for the conclusion that the CCSD(T)/CBS approach with high-level energy corrections can be used to provide reliable IE predictions for C(3)-C(7) hydrocarbon radicals with an uncertainty of +/-35 meV. Employing the atomization scheme, we have also computed the 0 K (298 K) heats of formation in kJ/mol at the CCSD(T)/CBS level for 2-C(3)H(7)/2-C(3)H(7) (+) ,C(6)H(5)/C(6)H(5) (+), and C(6)H(5)CH(2)/C(6)H(5)CH(2) (+) to be 105.2/822.7 (90.0/806.4), 351.4/1148.5 (340.4/1138.8), and 226.2/929.0 (210.3/912.7), respectively. Comparing these values with the available experimental values, we find that the discrepancies for the 0 and 298 K heats of formation values are < or =2.6 kJ/mol for 2-C(3)H(7)/2-C(3)H(7) (+),< or =4.1 kJ/mol for C(6)H(5)/C(6)H(5) (+), and < or =3.2 kJ//mol for C(6)H(5)CH(2)C(6)H(5)CH(2) (+).

A Combined Density Functional Theory and Coupled Cluster Method Investigation of the Structural Properties and Stabilities of Radical CH2CP and Its Isomers

The doublet potential energy surface of radical system [C(2), H(2), P] is investigated at the UB3LYP/6-311++G(d,p) and UCCSD(T)/6-311++G(2df,2p) (single-point) levels. Eight chainlike and three-membered ring structures are located as energy minima connected by 10 interconversion transition states. At the final UCCSD(T)/6-311++G(2df,2p)//UB3LYP/6-311++G(d,p) level with zero-point vibrational energy correction, species CH(2)CP is found to be thermodynamically the most stable isomer followed by HCCPH, H-cCPC-H, cPCC-H(H), H-cCCP-H, cis-CC(H)PH, trans-CC(H)PH, and CCPH(2) at 11.01, 12.57, 40.07, 43.63, 50.25, 56.82, and 65.36 kcal/mol, respectively. The computed results indicate that the chainlike isomers CH(2)CP and HCCPH and cyclic radical H-cCPC-H possess considerable kinetic stability at extra low pressures and temperatures. Interestingly, radical CCPH(2), whose energy is the highest in all predicted CH(2)CP isomers, can be also regarded as a kinetically stable species with the smallest isomerization barrier of 22.26 kcal/mol at extra low pressures and temperatures. Therefore, considering higher kinetic stability, in addition to the microwave spectroscopy characterized isomer CH(2)CP in previous experiments, the species HCCPH, H-cCPC-H, and CCPH(2) should be considered as excellent candidates for possible experimental observation. Furthermore, the structural nature of stable radical isomers is discussed based on bonding characteristics, single electron spin distribution, and comparison with their analogues.

Characterization of the X̃A12, ÃB12, and X̃Π2 electronic states of the Ga2H molecule and the X̃A′2 and ÃA″2 isomerization transition states connecting the three minima

A wide range of highly correlated ab initio methods has been used to predict the geometrical parameters of the linear (X (2)Pi) and H-bridged (X (2)A(1) and A (2)B(1)) Ga(2)H isomers and two isomerization transition states (X (2)A(') and A (2)A(")) connecting the three minima. Dipole moments and vibrational frequencies are also obtained. The global minimum X (2)A(1) ground state of the H-bridged GaHGa isomer is predicted to lie only 1.6 [1.9 with the zero-point vibrational energy (ZPVE) corrections] kcal mol(-1) below the A (2)B(1) state. The X (2)A(1) state lies 5.4 kcal mol(-1) below the X (2)Pi ground state of the linear GaGaH isomer at the coupled-cluster with single, double, and perturbative triple excitations [CCSD(T)] level of theory with the augmented correlation-consistent polarized valence quadruple-zeta (aug-cc-pVQZ) basis set. The full triples coupled-cluster method is found to alter these CCSD(T) predictions by as much as 0.3 kcal mol(-1). The forward isomerization barriers from the linear ground state to the X (2)A(') and A (2)A(") transition states are determined to be 3.3 and 5.3 kcal mol(-1), respectively. The reverse isomerization barrier between the X (2)A(1) GaHGa structure and the X (2)Pi GaGaH structure is predicted to be 8.6 (8.2 with the ZPVE corrections) kcal mol(-1) at the aug-cc-pVQZ CCSD(T) level of theory.

Reliable Electron Affinities of Perfluorocyclopropane and Perfluorocyclobutane from Convergent ab Initio Computations

To resolve discrepancies concerning the magnitude of the electron affinities of perfluorocyclopropane and perfluorocyclobutane, quantum chemical calculations have been carried out with the MP2 and CCSD(T) methods in conjunction with augmented correlation consistent basis sets (aug-cc-pVX Z, X = D, T, Q). Though no experimental values have been found for perfluorocyclopropane, we estimate its electron affinity to be 0.17 eV (0.00 eV without zero-point vibrational energy corrections). In addition, determination of the electron affinity of perfluorocyclobutane (0.61 and 0.44 eV with and without zero-point vibrational energy corrections, respectively) is in good agreement with experimental values reported by Miller and co-workers (0.63 +/- 0.05 eV). This study also demonstrates that the widely prescribed B3LYP/DZP++ model chemistry for computing electron affinities does not correctly describe these systems.

Conformational study of cationic, zwitterionic, anionic species of aspartic acid, water-added forms and their protonation. A DFT method

The potential energy surfaces of various species of aspartic acid have been computed at the DFT/B3LYP/6-31G(d) level of theory. Conformations of 34, 11, 9 and 3 were found from 324, 162, 81 and 81 possible conformers for species H 3asp +, H 2asp, Hasp − and asp 2−, respectively. The most stable conformation for species H 3asp +, H 2asp, Hasp − and asp 2− are γ D, α L, ε L and β L conformers, respectively. Three protonation steps due to the tetrahydrated forms of species asp 2−, Hasp − and H 2asp of aspartic acid computed at the B3LYP/6-31+G(d, p) level of theory with zero-point vibrational energy corrections are exothermic reactions and their stabilization energies of the stepwise protonation are −244.68, −308.57 and −379.97 kcal mol −1, respectively.

Intrinsic Conformational Preferences of Substituted Cyclohexanes and Tetrahydropyrans Evaluated at the CCSD(T) Complete Basis Set Limit: Implications for the Anomeric Effect

A series of MP2 and CCSD(T) computations have been carried out with correlation consistent basis sets as large as aug-cc-pV5Z to determine the intrinsic equatorial-axial conformational preference of CH(3)-, F-, OCH(3)-, and OH-substituted cyclohexane and tetrahydropyran rings. The high-accuracy relative electronic energies reported here shed new light on the intrinsic energetics of these cyclic prototypes for the anomeric effect. At the CCSD(T) complete basis set (CBS) limit, the energy of the equatorial conformation relative to the axial position (DeltaE (CBS)(CCSD(T))) is -1.75, -0.20, -0.21, and -0.56 kcal mol(-1) in methyl-, fluoro-, methoxy-, and hydroxycyclohexane, respectively, while DeltaE(CBS)(CCSD(T) is -2.83, +2.45, +1.27, and +0.86 kcal mol(-1) for 2-methyl-, 2-fluoro-, 2-methoxy-, and 2-hydroxytetrahydropyran, respectively. Note that the equatorial and axial conformers are nearly electronically isoenergetic in both fluoro- and methoxycyclohexane. For all eight cyclic species, a zero-point vibrational energy correction decreases Delta by a few tenths of a kilocalorie per mole. Relative energies obtained with popular methods and basis sets are unreliable, including Hartree-Fock theory, the B3LYP density functional, and the 6-31G and 6-311G families of split-valence basis sets. Even with the massive pentuple-zeta basis sets, the HF and B3LYP methods substantially overestimate the stability of the equatorial conformers (by as much as 0.99 and 0.73 kcal mol(-1), respectively, for 2-methoxytetrahydropyran). Only because of a consistent cancellation of errors do these popular approaches sometimes provide reasonable estimates of the anomeric effect.

Torsional potential of 4,4′-bipyridine: Ab initio analysis of dispersion and vibrational effects

Ab initio calculations using restricted Hartree-Fock, second-order Møller-Plesset perturbation theory (MP2), density-functional theory (DFT), and coupled-cluster methods have been done to obtain the torsional potential-energy profile of the aza-aromatic molecule 4,4'-bipyridine. The torsional potential is evaluated adiabatically by fixing the normalized sum of the dihedral angles through the C-C inter-ring bond at several values along the torsional path and relaxing the remaining degrees of freedom. Previous discrepancies between MP2 and DFT internal rotation barrier heights are removed, and seen to be mostly due to the underestimation of the dispersion energy in the coplanar conformer by MP2 when using relatively small basis sets. The calculations indicate that the barrier height between the twisted global minimum and the 0 degrees conformer is around 1.5-1.8 kcal mol-1 while that corresponding to the 90 degrees one is about 2.0-2.2 kcal mol-1. This same relative energy ordering of the coplanar and perpendicular conformers was experimentally derived from nuclear magnetic resonance (NMR) measurements of 1H dipolar couplings on 4,4'-bipyridine solutions in a nematic liquid crystal, although the barrier heights are much lower than those estimated from NMR experiments in the gas phase. The DFT infrared spectrum and zero-point vibrational energy corrections to the torsional energy profile have also been calculated, the latter having a small influence on the torsional potential-energy profiles.

Quantum Chemical Investigation of the Electronic Spectra of the Keto, Enol, and Keto−Imine Tautomers of Cytosine

The low-lying excited singlet states of the keto, enol, and keto-imine tautomers of cytosine have been investigated employing a combined density functional/multireference configuration interaction (DFT/MRCI) method. Unconstrained geometry optimizations have yielded out-of-plain distorted structures of the pi --> pi and n --> pi excited states of all cytosine forms. For the keto tautomer, the DFT/MRCI adiabatic excitation energy of the pi --> pi state (4.06 eV including zero-point vibrational energy corrections) supports the resonant two-photon ionization (R2PI) spectrum (Nir et al. Phys. Chem. Chem. Phys. 2002, 5, 4780). On its S1 potential energy surface, a conical intersection between the 1pipi state and the electronic ground state has been identified. The barrier height of the reaction along a constrained minimum energy path amounts to merely 0.2 eV above the origin and explains the break-off of the R2PI spectrum. The 1pipi minimum of the enol tautomer is found at considerably higher excitation energies (4.50 eV). Because of significant geometry shifts with respect to the ground state, long vibrational progressions are expected, in accord with experimental observations. For the keto-imine tautomer, a crossing of the 1pipi potential energy surface with the ground-state surface has been found, too. Its n --> pi minimum (3.27 eV) is located well below the conical intersection between the pi --> pi and S0 states, but it will be difficult to observe because of its small transition moment. The identified conical intersections of the pi --> pi excited states of the keto cytosine tautomers are made responsible for the ultrafast decay to the electronic ground states and thus may explain their subpicoseconds lifetimes.

Accurate prediction for electron affinities of the radicals derived from the halide benzene

The molecular structures and electron affinities of eight radicals derived from the halide benzene by removing a hydrogen atom have been determined using seven hybrid Hartree-Fock/density-functional methods. The basis set used in this work is of double-zeta plus polarization quality with additional diffuse s- and p-type functions, denoted as DZP++. These methods have been carefully calibrated [J. C. Rienstra-Kiracofe, G. S. Tschumper, H. F. Schaefer, S. Nandi, and G. B. Ellison, Chem. Rev. (Washington, D. C.) 102, 231 (2002)]. The geometries are fully optimized with each density-functional theory method and discussed, respectively. The three different types of the neutral-anion energy separations reported in this work are the adiabatic electron affinity, the vertical electron affinity, and the vertical detachment energy. The most reliable adiabatic electron affinities (with zero-point vibrational energy correction), obtained at the DZP++ B3LYP level of theory, are 1.74 eV (o-C6H4F), 1.39 eV (m-C6H4F), 1.34 eV (p-C6H4F), 1.78 eV (o-C6H4Cl), 1.53 eV (m-C6H4Cl), 1.45 eV (p-C6H4Cl), 2.06 eV (o-C6H3F2), and 2.04 eV (p-C6H3F2), respectively. Compared with the experimental values, the average absolute error of the B3LYP method is 0.03 eV. The BLYP, BP86, and BPW91 functionals also gave excellent predictions, with average absolute errors of 0.05, 0.08, and 0.08 eV, respectively.

Investigations of double proton transfer and one-electron oxidation behavior in double H-bonded glycine–formamide complex in the gas phase

The behaviors of double proton transfer (DPT) occurring in a representative glycine–formamide complex have been investigated employing the B3LYP/6-311++G** level of theory. The relevant quantities involved in the DPT process, such as interaction energies, tautomerization energy, equilibrium constant, and barrier heights have been discussed, respectively. Compared with the intramolecular proton transfer (PT) in isolated glycine, the participation of a formamide molecule favors the PT of glycine kinetically. The DPT process proceeds with a concerted mechanism rather than a stepwise one since no zwitterionic complexes have been located during the DPT process. Correspondingly, the calculated forward and reverse barrier heights are 12.62 and 2.01 kcal/mol, respectively. However, both of them have been reduced to 9.35 and −1.16 kcal/mol when zero-point vibrational energy (ZPVE) corrections are included, where the disappearance of the reverse barrier height implies that the reverse reaction should proceed with barrierless spontaneously. Additionally, the one-electron oxidation behavior for the double H-bonded glycine–formamide complex has also been investigated. The oxidation product is characterized by a distonic radical cation due to the fact that one-electron oxidation takes place on glycine fragment and a proton has been transferred from glycine to formamide fragment spontaneously. As a result, the vertical and adiabatic ionization potentials for the neutral complex have been determined to be about 9.20 and 8.26 eV, respectively.

Application of equation-of-motion coupled-cluster methods to low-lying singlet and triplet electronic states of HBO and BOH

The equilibrium structures and physical properties of the X (1)sigma(+) linear electronic states, linear excited singlet and triplet electronic states of hydroboron monoxide (HBO) (A (1)sigma(-), B (1)delta, a (3)sigma(+), and b (3)delta) and boron hydroxide (BOH) (A (1)sigma(+), B (1)Pi, and b (3)Pi), and their bent counterparts (HBO a (3)A('), b (3)A("), A (1)A("), B (1)A(') and BOH X (1)A('), b (3)A('), c (3)A("), A (1)A('), B (1)A('), C (1)A(")) are investigated using excited electronic state ab initio equation-of-motion coupled-cluster (EOM-CC) methods. A new implementation of open-shell EOM-CC including iterative partial triple excitations (EOM-CC3) was tested. Coupled-cluster wave functions with single and double excitations (CCSD), single, double, and iterative partial triple excitations (CC3), and single, double, and full triple excitations (CCSDT) are employed with the correlation-consistent quadruple and quintuple zeta basis sets. The linear HBO X (1)sigma(+) state is predicted to lie 48.3 kcal mol(-1) (2.09 eV) lower in energy than the BOH X (1)sigma(+) linear stationary point at the CCSDT level of theory. The CCSDT BOH barrier to linearity is predicted to lie 3.7 kcal mol(-1) (0.16 eV). With a harmonic zero-point vibrational energy correction, the HBO X (1)sigma(+)-BOH X (1)A(') energy difference is 45.2 kcal mol(-1) (1.96 eV). The lowest triplet excited electronic state of HBO, a (3)A('), has a predicted excitation energy (T(e)) of 115 kcal mol(-1) (4.97 eV) from the HBO ground state minimum, while the lowest-bound BOH excited electronic state, b (3)A('), has a T(e) of 70.2 kcal mol(-1) (3.04 eV) with respect to BOH X (1)A('). The T(e) values predicted for the lowest singlet excited states are A (1)A(")<--X (1)sigma(+)=139 kcal mol(-1) (6.01 eV) for HBO and A (1)A(')<--X (1)A(')=102 kcal mol(-1) (4.42 eV) for BOH. Also for BOH, the triplet vertical transition energies are b (3)A(')<--X (1)A(')=71.4 kcal mol(-1) (3.10 eV) and c (3)A(")<--X (1)A(')=87.2 kcal mol(-1) (3.78 eV).

Accurate ab initio predictions of ionization energies of hydrocarbon radicals: CH2, CH3, C2H, C2H3, C2H5, C3H3, and C3H5

The ionization energies for methylene (CH2), methyl (CH3), ethynyl (C2H), vinyl (C2H3), ethyl (C2H5), propargyl (C3H3), and allyl (C3H5) radicals have been calculated by the wave-function-based ab initio CCSD(T)/CBS approach, which involves the approximation to the complete basis set (CBS) limit at the coupled-cluster level with single and double excitations plus a quasiperturbative triple excitation [CCSD(T)]. When it is appropriate, the zero-point vibrational energy correction, the core-valence electronic correction, the scalar relativistic effect correction, the diagonal Born-Oppenheimer correction, and the high-order correlation correction have also been made in these calculations. The comparison between the computed ionization energy (IE) values and the highly precise experimental IE values determined in previous pulsed field ionization-photoelectron (PFI-PE) studies indicates that the CCSD(T)/CBS method is capable of providing accurate IE predictions for these hydrocarbon radicals achieving error limits well within +/-10 meV. The benchmarking of the CCSD(T)/CBS IE predictions by the PFI-PE experimental results also lends strong support for the conclusion that the CCSD(T)/CBS approach with high-level energy corrections can serve as a valuable alternative for reliable IE determination of radicals, particularly for those radicals with very unfavorable Franck-Condon factors for photoionization transitions near their ionization thresholds.

Double proton transfer and one‐electron oxidation behavior in double H‐bonded glycinamide–glycine complex in the gas phase

The behaviors of double proton transfer (DPT) occurring in a representative glycinamide-glycine complex have been investigated employing the B3LYP/6-311++G** level of theory. Thermodynamic and especially kinetic parameters, such as tautomerization energy, equilibrium constant, and barrier heights have been discussed, respectively. The relevant quantities involved in the DPT process including geometrical changes, interaction energies, and deformation energies have also been studied. Analogous to that of tautomeric process assisted with a formic acid molecule, the participation of a glycine molecule favors the proceeding of the proton transfer (PT) for glycinamide compared with that without mediator-assisted case. The DPT process proceeds with a concerted mechanism rather than a stepwise one because no zwitterionic complexes have been located during the DPT process. The barrier heights are 12.14 and 0.83 kcal/mol for the forward and reverse directions, respectively. However, both of them have been reduced by 3.10 and 2.66 kcal/mol to 9.04 and -1.83 kcal/mol with further inclusion of zero-point vibrational energy (ZPVE) corrections, where the disappearance of the reverse barrier height implies that the reverse reaction should proceed with barrierless spontaneously, analogous to those of DPTs occurring between glycinamide and formic acid (or formamide). Additionally, the oxidation process for the double H-bonded glycinamide-glycine complex has also been investigated. The oxidated product is characterized by a distonic radical cation due to the fact that one-electron oxidation takes place on glycine fragment and a proton has been transferred from glycine to glycinamide fragment spontaneously. As a result, the vertical and adiabatic ionization potentials for the neutral complex have been determined to be about 8.71 and 7.85 eV, respectively, where both of them have been reduced by about 0.54 (1.11) and 0.75 (1.13) eV relative to those of isolated glycinamide (glycine) due to the formation of the intermolecular H-bond.