MP2 Level Research Articles

The torsional spectrum of the hydrogen trioxide molecule

The energies of the stationary torsional levels of the hydrogen trioxide molecule were calculated at the B3LYP, MP2, and CCSD(T) levels of theory using augmented correlation consistent acc-pVTZ basis set. The molecular symmetry group whose elements are inherent to both equilibrium conformer's symmetry elements (C2 and CS) of the HT molecule was found. Different methods of the molecular parameters calculations were suggested and analyzed. The torsional, spin and total wave functions were classified by irreducible representations of the C2V(M) molecular symmetry group. The 2D dipole moment surface was calculated too. The energies of the stationary torsional states were found using DVR and Fourier methods. With this and acceptable combinations of spatial and spin wave functions, the IR torsional spectrum was calculated at different temperatures. The tunneling frequencies in the ground and some excited torsional states were estimated too.

Determination of a semi-experimental equilibrium structure of 1-phosphapropyne from millimeter-wave spectroscopy of [formula omitted] and [formula omitted

Trideuterated 1-phosphapropyne (CD3CP) has been produced by co-pyrolysis of phosphorus trichloride and esadeuterated ethane. The rotational spectra of CD3CP in the ground and the low-lying vibrational states v8 (CCP bending mode) and 2 v8 have been investigated in the millimeter-wave region. Very accurate values of the quartic centrifugal distortion constants DJ and DJK and of the sextic distortion constants HJK and HKJ have been obtained for the ground state. l-type resonance effects have been taken into account in the analysis of the spectra of the degenerate bending states, so that the energy difference between the v8|l| = 20, and v8|l| = 22 states could be determined, together with a number of spectroscopic constants involved in the l-type resonance. Moreover, for the parent isotopologue CH3CP a new set of excited-state lines (v8 = 1, 2 and v4 = 1) were recorded up to 330 GHz, and a global analysis including all available rotational and rovibrational transitions related to the ground and v8 = 1, 2 states has been performed. A satisfactory fit could only be obtained by making a reassignment of the few transition wavenumbers previously measured for the 2ν80←ν8±1 hot band [M.K. Bane et al., J. Mol. Spectrosc.275 (2012) 9–14].The experimental work has been combined with quantum-chemical computations. Quadratic and cubic force constants of 1-phosphapropyne have been calculated at MP2 level of theory with a basis set of triple-ζ quality. A semi-experimental equilibrium structure has been derived by correcting experimental ground-state rotational constants by means of theoretical vibration-rotation interaction constants.

Theoretical Investigation of Cyano‐Chalcogen Dimers and Their Importance in Molecular Recognition

In this manuscript the different noncovalent interactions established between (HYCN)2 dimers (Y=S, Se and Te) have been studied at the MP2 and CCSD(T) level of theory. Several homodimers have been taken into account, highlighting the capacity of these compounds to act both as electron donor and acceptor. The main properties studied were geometries, binding energy (Eb ), and molecular electrostatic potential (MEP). Given the wide application of chalcogen bonds, and more specifically of cyano-chalcogen moieties in molecular recognition, natural bond orbital (NBO), "atoms-in-molecules" (AIM), and electron density shift (EDS) analysis were also used to analyse the different noncovalent interactions upon complexation. The presence of hydrogen, chalcogen and dipole-dipole interactions was confirmed and their implications on molecular recognition were analysed.

Energy Decomposition Analysis for Interactions of Radicals: Theory and Implementation at the MP2 Level with Application to Hydration of Halogenated Benzene Cations and Complexes between CO2-· and Pyridine and Imidazole.

To study intermolecular interactions involving radicals at the correlated level, the energy decomposition analysis scheme for second-order Mo̷ller-Plesset perturbation theory based on absolutely localized molecular orbitals (ALMO-MP2-EDA) is generalized to unrestricted and restricted open-shell MP2. The benefit of restricted open-shell MP2 is that it can provide accurate binding energies for radical complexes where density functional theory can be error-prone due to delocalization errors. As a model application, the open-shell ALMO-MP2-EDA is applied to study the first solvation step of halogenated benzene radical cations, where both halogen- and hydrogen-bonded isomers are possible. We determine that the lighter halogens favor the hydrogen-bonded form, while the iodine-substituted species prefers halogen bonding due to larger polarizability and charge transfer at the halogen. As a second application, relevant to the activation of CO2 in photoelectrocatalysis, complexes of CO2-· interacting with both pyridine and imidazole are analyzed with ALMO-MP2-EDA. The results reveal the importance of charge transfer into the π* orbital of the heterocycle in controlling the stability of the carbamate binding mode, which is favored for pyridine but not for imidazole.

Predicting Stable Molecular Structures for (RNC)2AuIX Complexes

Calculations have been performed at the MP2 and DFT levels for investigating the reasons for the difficulties in synthesizing bis(isocyanide)gold(I) halide complexes. Three‐coordinated gold(I) complexes of the type (R3P)2AuIX (1) can be synthesized, whereas the analogous isocyanide complexes (RNC)2AuIX (2) are not experimentally known. The molecular structures of (R3P)2AuIX (X = Cl, Br, and I) and (RNC)2AuIX with X = halide, cyanide, nitrite, methylthiolate, and thiocyanate are compared and structural differences are discussed. Calculations of molecular properties elucidate which factors determine the strength of the gold‐ligand interactions in (RNC)2AuIX. The linear bonding mode of RNC favors a T‐shaped geometry instead of the planar Y‐shaped trigonal structure of (R3P)2AuIX complexes that have been synthesized. An increased polarity of the Au–X bond in 2 leads to destabilization of the Y‐shaped structure. Chalcogen‐containing ligands or cyanide appear to be good X‐ligand candidates for synthesis of (RNC)2AuIX complexes.

Ion Pairing in HCl-Water Clusters: From Electronic Structure Investigations to Multiconfigurational Force-Field Development.

In the bulk, condensed-phase HCl exists as a dissociated Cl- ion and a proton that is delocalized over solvating water molecules. However, in the gas phase, HCl is covalent, and even on the introduction of hydrating water molecules, the HCl covalent state dominates small clusters and is relevant at larger clusters including 21 water molecules. Electronic structure calculations (at the MP2 level) and ab initio metadynamics simulations (at the DFT level) have been carried out on HCl-(H2O)n clusters with n = 2-22 to investigate distinct solvation environments in clusters from covalent HCl structure, to contact ion pairs and solvent-separated ion pairs. The data were further used to train and validate a multiconfigurational force-field for HCl-water clusters that incorporates covalent HCl states into the MS-EVB3.2 formalism. Additionally, the many-body interaction of the Cl- ion with water and the excess proton was modeled by the introduction of two geometric three-body terms that incorporates the dominant many-body interaction in an efficient noniterative manner.

1-Methylthio-1-phenyl-1-silacyclohexane: Synthesis, conformational preferences in gas and solution by GED, NMR and theoretical calculations

1-Methylthio-1-phenyl-1-silacyclohexane 1, the first silacyclohexane with the sulfur atom at silicon, was synthesized and its molecular structure and conformational preferences studied by gas-phase electron diffraction (GED) and low temperature 13C and 29Si NMR spectroscopy (LT NMR). Quantum-chemical calculations were carried out both for the isolated species and solvate complexes in gas and in polar medium. The predominance of the 1-MeSaxPheq conformer in gas phase (1-Pheq:1-Phax = 55:45, ΔG° = 0.13 kcal/mol) determined from GED is consistent with that measured in the freon solution by LT NMR (1-Pheq:1-Phax = 65:35, ΔG° = 0.12 kcal/mol), the experimentally measured ratios being close to that estimated by quantum chemical calculations at both the DFT and MP2 levels of theory.

Low torsional barrier challenges in the microwave spectrum of 2,4-dimethylanisole.

Low barriers to internal rotations are especially challenging for both the experimental and theoretical determinations because they result in large tunneling splittings which are hard to assign and in potential functions that can be difficult to model. In the present work, the internal rotations of two methyl groups of 2,4-dimethylanisole were analyzed and modeled using a newly developed computer code, called ntop, adapted for fitting the high-resolution torsion-rotation spectra of molecules with two or more methyl rotors. The spectrum was measured using a pulsed molecular jet Fourier transform microwave spectrometer operating in the frequency range of 2.0-26.5 GHz, revealing internal rotation tunneling quintets with splittings of up to several gigahertz. The V3 potential barriers are 441.139(23) cm-1 and 47.649(30) cm-1 for the o- and p-methyl groups, respectively. Quantum chemical calculations predicted only one conformer with the methoxy group in the anti position related to the neighboring o-methyl group. While the results from geometry optimizations were reliable, ab initio calculations at the MP2 level did not reproduce the low torsional barriers, calling for further experiments on related systems and additional theoretical models.

Complexes of Glycolic Acid with Nitrogen Isolated in Argon Matrices. I. Structures and Thermal Effects.

Molecular complexes between glycolic acid and nitrogen were studied in a low-temperature argon matrix with FTIR spectroscopy, and supported by MP2 and BLYPD3 calculations. The calculations indicate 11 and 10 stable complex structures at the MP2 and BLYPD3 levels of theories, respectively. However, only one hydrogen-bonded complex structure involving the most stable SSC conformer of glycolic acid was found experimentally, where the nitrogen molecule is bound with the carboxylic OH group of the SSC conformer. The complex shows a rich site structure variation upon deposition of the matrix in different temperatures and upon annealing experiments, which provide interesting prospects for site-selective chemistry.

The analysis of the Coriolis interactions in the v10 = 1, v7 = 1 and v4 = 1 triad rovibrational states of 13C2D4 by high-resolution FTIR spectroscopy

ABSTRACTThe high-resolution Fourier transform infrared spectra of the B-type and C-type bands of 13C2D4 were recorded for the first time at a resolution of 0.0019 cm−1 in the 515–825 cm−1 region. The , and states are coupled through a-, b- and c-axes Coriolis resonances. Local perturbations between and rendered some transitions in the infrared inactive band sufficiently intense for analysis. A total of 6379 infrared transitions (4684 of , 1511 of and 184 of ) were assigned for the first time and fitted with an rms deviation of 0.00042 cm−1, with a Hamiltonian containing Watson-type rotational terms in the representation and A-reduction in the diagonal blocks and eight off-diagonal Coriolis terms to derive precise rovibrational constants for the upper states. Ground state spectroscopic constants up to sextic terms were also derived from a fit of 5266 ground state combination differences (GSCDs) with an rms deviation of 0.00045 cm−1, from infrared transitions of the present analysis and those determined previously. The experimental constants of the ground and upper states compared favourably to the theoretically calculated constants using the cc-pVQZ basis set at MP2 and B3LYP levels.

Calculation of 15N NMR Chemical Shifts in a Diversity of Nitrogen-Containing Compounds Using Composite Method Approximation at the DFT, MP2, and CCSD Levels.

Computations of 15N NMR chemical shifts in 93 diverse nitrogen-containing compounds representing almost all known classes are performed at the density functional theory (DFT), second-order Møller-Plesset perturbation theory (MP2), and coupled cluster singles and doubles (CCSD) levels using the composite method approximation (CMA) in comparison with experimental results. It is shown that the CMA-DFT and CMA-CCSD methods provided the best performance characterized by a normalized mean absolute error of 1.1-1.3% as compared to 2.3% for the CMA-MP2 results. Taking into account solvent effects within the conductor-like polarizable continuum model decreased the normalized mean absolute error by 0.4% for the CMA-DFT and by 0.2% for the CMA-CCSD calculations.

Accuracy evaluation and addition of improved dihedral parameters for the MMFF94s

The Platinum dataset of protein-bound ligand conformations was used to benchmark the ability of the MMFF94s force field to generate bioactive conformations by minimization of randomly generated conformers. Torsion angle parameters that generally caused wrong geometries were reparameterized by conducting dihedral scans using ab initio calculations at the MP2 level. This reparameterization resulted in a systematic improvement of generated conformations.

Cl-Atom-Initiated Atmospheric Degradation of Saturated Cyclic Hydrocarbons: Kinetic and Mechanistic Investigation.

The temperature-dependent reaction kinetics of methyl cyclohexane (MCH) and methyl cyclopentane (MCP) with Cl atoms were experimentally explored via the relative rate technique. Gas chromatography coupled with a flame ionization detector was employed to follow the reactant as well as the reference compound concentrations during the course of reaction. Gas chromatography coupled with mass spectrometry and gas chromatography coupled with infrared spectroscopy were used as the diagnostic tools for the detection and identification of the products in the title reaction. The rate coefficients for the reaction of Cl atoms with methyl cyclohexane and methyl cyclopentane were measured in the temperature range of 283-363 K at 760 Torr using isoprene and propylene as reference compounds. To support the experimental results, computational calculations were performed over the temperature range of 200-400 K using canonical variational transition state theory coupled with small curvature tunneling (CVT/SCT), conventional transition state theory (CTST) coupled with Wigner tunneling corrections, and CVT coupled with Wigner tunneling methods using the MP2 level of theory with 6-31G(d,p) as the basis set. The temperature-dependent rate coefficients were measured for the reaction of methyl cyclohexane and methyl cyclopentane with Cl atoms to be k(MCH) = [(4.48 ± 0.75) × 10-11]exp[(604 ± 25)/T] cm3 molecule-1 s-1 and k(MCP) = [(5.71 ± 0.66) × 10-12]exp[(1819 ± 669)/T] cm3 molecule-1 s-1, respectively. The measured rate coefficients at 298 K for the reactions of Cl atoms with methyl cyclohexane and methyl cyclopentane are k298KMCH = (3.36 ± 0.34) × 10-10 cm3 molecule-1 s-1 and k298KMCP = (2.25 ± 0.24) × 10-10 cm3 molecule-1 s-1, respectively. The conceivable atmospheric degradation mechanism for the reaction of methyl cyclohexane as well as methyl cyclopentane with Cl atoms was projected based on the products obtained during the reaction. Atmospheric implications, cumulative atmospheric lifetimes, thermochemistry, ozone formation potentials, and branching ratios of these molecules were also calculated and have been reported in this article.

High-resolution Fourier transform infrared spectrum of the [formula omitted] band of formamide-d1 (DCONH2)

The high-resolution rovibrational spectrum of the ν6 band of formamide-d1 (DCONH2) was recorded using the Fourier transform infrared (FTIR) spectrometer at the Australian Synchrotron with an unapodized resolution of 0.00142 cm−1 in the mid-infrared region of 910–990 cm−1 region. From the rovibrational analysis of this A-type band, the v6 = 1 state rovibrational constants up to one sextic term were derived for the first time from a total of 1450 infrared transitions. These transitions were fitted using the Watson's A-reduced Hamiltonian in the Ir representation with a root-mean-square (rms) deviation of 0.00078 cm−1. The center of the ν6 band of DCONH2 was accurately determined to be 954.857070(41) cm−1. This v6 = 1 state was found to be perturbed by the nearby (v10=1,v12 = 1) state, just around 16 cm−1 higher. Two c-Coriolis parameters were obtained from the perturbation analysis of the interaction between the rotational energy levels of the v6 = 1 and (v10=1,v12 = 1) states. Additionally, the center of the ν10+ν12 band was found at 970.671(41) cm−1 and rotational constant C was fitted accurately. Ground state rotational constants and centrifugal distortion constants up to two sextic terms were obtained by fitting of 1432 ground state combination differences (GSCDs) derived from the infrared (IR) transitions of both the ν6 band of the present work and ν12 band of previous work of DCONH2, together with 6 previously reported microwave frequencies. The root-mean-square (rms) deviation value of this fit was 0.000183 cm−1. The ground state constants in this study provide a better representation for the DCONH2 isotopologue as the values were derived from a larger pool of GSCDs. In addition, ground state rovibrational constants up to five quartic terms and rotational constants of the v6 = 1 state were computed from theoretical anharmonic calculations at two different levels of theory, B3LYP and MP2 with the cc-pVTZ basis set, for comparison with the experimental results.

Toward Closing the Gap between Hexoses and N-Acetlyhexosamines: Experimental and Computational Studies on the Collision-Induced Dissociation of Hexosamines.

Motivated by the fundamental difference in the reactivity of hexoses and N-acetylhexosamines under collision-induced dissociation (CID) mass spectrometry conditions, we have investigated the CID of two hexosamines, glucosamine (GlcN) and galactosamine (GalN), experimentally and computationally. Both hexosamines undergo ring-opening and then dissociate via the 0,2A and the 0,3A (0,3X) cross-ring cleavage channels. The preference for the ring-opening is similar to the behavior of N-acetylhexosamines and explains why the two anomers of the same sugar give the same mass spectrum. While the spectrum for GlcN is dominated by the 0,2A signal, the signal intensities for both 0,2A and the 0,3A (0,3X) dissociation channels are comparable for GalN, which allows GlcN and GalN to be distinguished easily. Calculations at MP2 level of theory indicate that this is related to the differences in the relative barrier heights for the 0,2A and the 0,3A (0,3X) cross-ring cleavage channels. This, in return, reflects the circumstance that the 0,2A cross-ring cleavage barriers are different for the two sugars, while the barriers of all other dissociation channels are comparable. While the mechanisms of the cross-ring dissociation channels of hexoses are well described using the retro-aldol mechanism in the literature, this study proposes a new mechanism for the 0,3A (0,3X) cross-ring cleavage of hexosamines that involves the formation of an epoxy intermediate or a zwitterionic intermediate.

Infrared Intensification and Hydrogen Bond Stabilization: Beyond Point Charges.

Infrared band intensification of the A-H bond stretching mode of A-H···B acid-base systems has long been known to be the most spectacular spectral change occurring on hydrogen bonding. A QTAIM/CCTDP model is reported here to quantitatively explain the electronic structure origins of intensification and investigate the correlation between experimental enthalpies of formation and infrared hydrogen bond stretching intensifications amply reported in the literature. Augmented correlation-consistent polarized triple-zeta quantum calculations at the MP2 level were performed on complexes with HF and HCl electron acceptors and HF, HCl, NH3, H2O, HCN, acetonitrile, formic acid, acetaldehyde, and formaldehyde electron donor molecules. The A-H stretching band intensities are calculated to be 3 to 40 times larger than their monomer values. Although the acidic hydrogen atomic charge is important for determining the intensities of HF complexes relative to HCl complexes with the same electron donor, they are not important for infrared intensifications occurring on hydrogen bond formation for a series of bases with a common acid. Charge transfers are found to be the most important factor resulting in the intensifications, but dipolar polarization effects are also significant for each series of complexes. A mechanism involving intra-acid and intermolecular electron transfers as well as atomic polarizations is proposed for understanding the intensifications. The calculated sums of the intermolecular electron transfer and acid dipolar polarization contributions to the dipole moment derivatives for each series of complexes are highly correlated with their enthalpies of formation and H-bond intensifications. This could be related to increasing electron transfer from base to acid that correlates with the calculated hydrogen bonding energies and may be a consequence of the A-H bond elongation on complex formation having amplitudes similar to those expected for the A-H vibration.

Data-Driven Acceleration of the Coupled-Cluster Singles and Doubles Iterative Solver.

Solving the coupled-cluster (CC) equations is a cost-prohibitive process that exhibits poor scaling with system size. These equations are solved by determining the set of amplitudes (t) that minimize the system energy with respect to the coupled-cluster equations at the selected level of truncation. Here, a novel approach to predict the converged coupled-cluster singles and doubles (CCSD) amplitudes, thus the coupled-cluster wave function, is explored by using machine learning and electronic structure properties inherent to the MP2 level. Features are collected from quantum chemical data, such as orbital energies, one-electron Hamiltonian, Coulomb, and exchange terms. The data-driven CCSD (DDCCSD) is not an alchemical method because the actual iterative coupled-cluster equations are solved. However, accurate energetics can also be obtained by bypassing solving the CC equations entirely. Our preliminary data show that it is possible to achieve remarkable speedups in solving the CCSD equations, especially when the correct physics are encoded and used for training of machine learning models.

The directional bonding of [1.1.1]propellane with next generation QTAIM

We investigated the [1.1.1]propellane molecule using the 3-D bond-path framework set B and the stress tensor Bσ within the quantum theory of atoms in molecules (QTAIM). The controversial axial bond was determined to be a charge-shift bond comprising significant bond metallicity that correlated with values of the bond stiffness S < 1. The influence of the axial bond on the neighboring bonding is quantified in terms of unexpectedly low bond stiffness S values and a new measure of charge-shift bonding, the polarizability P. Consistency of these results was found at the MP2, CCSD and B3LYP theory levels.

A computational study on the characteristics of open-shell H-bonding interaction between carbamic acid (NH2COOH) and HO2, HOS or HSO radicals.

Quantum chemical computations were applied to investigate the characteristics of open-shell hydrogen-bonding interactions in the complexes of carbamic acid (NH2COOH, CA) with HO2, HOS and HSO radicals. All the resulting complexes were studied using the MP2, B3PW91 and B3LYP computational levels and 6311++G** basis set. Geometry optimizations show that the O-H⋯O contact is stronger than N-H⋯O and S-H⋯O. The interaction energies revealed that all the radicals form stronger hydrogen bonded complexes at site-1, as confirmed by electron-density (ρ) and corresponding Laplacian (∇2ρ) values obtained by atoms in molecule (AIM) analysis. Non-covalent interaction and reduced density gradient analysis support the AIM results. Natural bond orbital analysis was employed to obtain the stabilization energies (E(2)) due to charge delocalization between the interacting units. Energy decomposition analysis suggests that, for the title complexes, the exchange energy makes a larger contribution to the total interaction energy compared to other energy terms. Graphical abstract Open-shell H-bondinginteraction between carbamic acid (NH2COOH) and HO2, HOS or HSOradicals.

Spectra-Structure Correlations in Isotopomers of Ethanol (CX3CX2OX; X = H, D): Combined Near-Infrared and Anharmonic Computational Study.

The effect of isotopic substitution on near-infrared (NIR) spectra has not been studied in detail. With an exception of few major bands, it is difficult to follow the spectral changes due to complexity of NIR spectra. Recent progress in anharmonic quantum mechanical calculations allows for accurate reconstruction of NIR spectra. Taking this opportunity, we carried out a systematic study of NIR spectra of six isotopomers of ethanol (CX3CX2OX; X = H, D). Besides, we calculated the theoretical spectra of two other isotopomers (CH3CD2OD and CD3CH2OD) for which the experimental spectra are not available. The anharmonic calculations were based on generalized vibrational second-order perturbation theory (GVPT2) at DFT and MP2 levels with several basis sets. We compared the accuracy and efficiency of various computational methods. It appears that the best results were obtained with B2PLYP-GD3BJ/def2-TZVP//CPCM approach. Our simulations included the first and second overtones, as well as binary and ternary combinations bands. This way, we reliably reproduced even minor bands in the spectra of diluted samples (0.1 M in CCl4). On this basis, the effect of isotopic substitution on NIR spectra of ethanol was accurately reproduced and comprehensively explained.