MP2 Level Research Articles

FT-IR and FT-Raman spectra of 5-chlorocytosine: Solid state simulation and tautomerism. Effect of the chlorine substitution in the Watson-Crick base pair 5-chlorodeoxycytidine-deoxyguanosine

The laser Raman and IR spectra of 5-chlorocytosine have been recorded and accurately assigned in the solid state using Density functional calculations (DFT) together with the linear scaling equation procedure (LSE) and the solid state simulation of the crystal unit cell through a tetramer form. These results remarkably improve those reported previously by other authors. Several new scaling equations were proposed to be used in related molecules. The six main tautomers of the biomolecule 5-chlorocytosine were determined and optimized at the MP2 and CCSD levels, using different basis sets. The relative stabilities were compared with those obtained in cytosine and their 5-halo derivatives. Several relationships between energies, geometric parameters and NBO atomic charges were established. The effect of the chlorine substitution in the fifth position was evaluated through the stability of the Watson-Crick (WC) base pair of 5-chlorodeoxycytidine with deoxyguanosine, and through their vibrational spectra.

Anharmonic vibrational and electronic spectral study of 2-amino-4-hydroxy-6–methylpyrimidine: A combined experimental (FTIR, FT-Raman, UV–Vis) and theoretical (DFT, MP2) approach

A combined experimental and theoretical study of the structure, vibrational spectra and electronic spectra of 2-amino-4-hydroxy-6-methylpyrimidine in the ground electronic state are reported. Anharmonic frequencies for the most stable conformer have been simulated using GVPT2, VSCF and PT2-VSCF methods with potential energy surface calculated using MP2 and DFT level of theory with 6-311G(d,p) basis set. The vibrational spectra (FTIR and FT-Raman) are interpreted in terms of fundamental, combination and overtone bands. It is found that the experimental and the VPT2 computed frequencies are in well agreement. The experimental and the calculated UV–Vis spectrum in gas and ethanol solvent are found comparable. Furthermore, HOMO–LUMO analysis, NLO, MEP, and natural charges of the molecule are also reported.

Intermolecular interactions in group 14 hydrides: Beyond CH···HC contacts

In line with previous work in which we established the factors that enhance attractive CH···HC dihydrogen interactions in alkanes, an extended theoretical analysis of noncovalent intermolecular interactions in group 14 hydrides is presented here. Remarkably, these weak interactions may play a major role in determining the crystal structures adopted by several families of molecules. A combined structural and computational analysis at the MP2 level allowed us to identify and characterize different interactions of the type EH···HE and E···HE (E = Si, Ge, Sn, and Pb), and to find also the most suitable scenario for the establishment of each particular type. The nature of the interactions has been analyzed in terms of natural charges of the atoms involved and a topological analysis of the electron density of several dimers confirms the existence of H···H and H···E attractive contacts. We have observed that the interaction strength increases when descending down the periodic group and that silicon has a marked tendency to establish Si···HSi interactions. A size-dependent backbone effect that reinforces H···H dihydrogen interactions in polyhedral systems has also been found.

The geometric and electronic structures of AlnNam (n = 5, 6; n + m ≤ 10) clusters

The low-energy structures of Al5Nam (m=1–5) and Al6Nam (m=0–4) are searched by using the genetic algorithm combined with MOPAC package. The stabilities and the electronic structures are studied by density functional theory models and by first principle perturbation theory at MP4 level. The results show that the Na atoms are located at the outside of the clusters. Analysis on the density of states and the charge distribution indicate the sodium atoms transfer their 3s electrons to aluminum and exist as +1 cations. The molecular orbitals agree with the types predicted by the jellium model, and Al5Na5 and Al6Na2 show enhanced stabilities as the 20 valence electrons form closed electronic shells. However, the valence orbitals don’t encompass the Na+ cores. The electron localization function clearly demonstrates that the valence electrons reside at the outside of the Al3+ cores, and the Na+ cores are exposed at the outside of the valence electrons. All the results confirm that Aln moieties in the AlNa binary clusters form Zintl anions and the Na+ cations are ionically bonded to them. It suggests AlNa clusters can be used as building blocks to form cluster-assembled Zintl-phase materials.

Favored Conformations of Carbonyl Compounds: A Structural Study of n-Octanal.

We report on the molecular structures of the two most abundant conformers of n-octanal observed by molecular beam Fourier transform microwave spectroscopy. Next to limonene, which is the main component of citrus-oil, octanal and other n-alkyl aldehydes strongly enhance the typical fresh smell of lemon-oil. Due to the high flexibility of its n-alkyl chain and the high number of possible conformers, different semi-empirical methods (AM1, PM3, MMFF94) were used to sample the conformational space of octanal before performing more sophisticated quantum chemical calculations at the MP2 level of theory. This technique has previously been shown to be an ideal tool to characterize relevant odorant structures in fragrance chemistry. The structure of octanal and structurally related molecules is discussed in the context of the most abundant chain conformations and the potential use of the microwave validated structures for further studies in biological media.

Ionization of pyridine: Interplay of orbital relaxation and electron correlation.

The valence shell ionization spectrum of pyridine was studied using the third-order algebraic-diagrammatic construction approximation scheme for the one-particle Green's function and the outer-valence Green's function method. The results were used to interpret angle resolved photoelectron spectra recorded with synchrotron radiation in the photon energy range of 17-120 eV. The lowest four states of the pyridine radical cation, namely, 2A2(1a2-1), 2A1(7a1-1), 2B1(2b1-1), and 2B2(5b2-1), were studied in detail using various high-level electronic structure calculation methods. The vertical ionization energies were established using the equation-of-motion coupled-cluster approach with single, double, and triple excitations (EOM-IP-CCSDT) and the complete basis set extrapolation technique. Further interpretation of the electronic structure results was accomplished using Dyson orbitals, electron density difference plots, and a second-order perturbation theory treatment for the relaxation energy. Strong orbital relaxation and electron correlation effects were shown to accompany ionization of the 7a1 orbital, which formally represents the nonbonding σ-type nitrogen lone-pair (nσ) orbital. The theoretical work establishes the important roles of the π-system (π-π* excitations) in the screening of the nσ-hole and of the relaxation of the molecular orbitals in the formation of the 7a1(nσ)-1 state. Equilibrium geometric parameters were computed using the MP2 (second-order Møller-Plesset perturbation theory) and CCSD methods, and the harmonic vibrational frequencies were obtained at the MP2 level of theory for the lowest three cation states. The results were used to estimate the adiabatic 0-0 ionization energies, which were then compared to the available experimental and theoretical data. Photoelectron anisotropy parameters and photoionization partial cross sections, derived from the experimental spectra, were compared to predictions obtained with the continuum multiple scattering approach.

Nitroxide stable radicals interacting as Lewis bases in hydrogen bonds: A search in the Cambridge structural data base for intermolecular contacts

1125 X-ray structures of nitroxide free radicals presenting intermolecular hydrogen bonds have been reported in the Cambridge Structural Database. We will report in this paper a qualitative and quantitative analysis of these bonds. The observation in some plots of an excluded region was statistically analyzed using convex hull and kernel smooting methodologies. A theoretical study at the MP2 level with different basis has been carried out indicating that the nitronyl nitroxide radicals (five electrons) lie just in between nitroso compounds (four electrons) and amine N-oxides (six electrons) as far as hydrogen-bond basicity is concerned.

Conventional and Explicitly Correlated ab Initio Benchmark Study on Water Clusters: Revision of the BEGDB and WATER27 Data Sets.

Benchmark ab initio energies for BEGDB and WATER27 data sets have been re-examined at the MP2 and CCSD(T) levels with both conventional and explicitly correlated (F12) approaches. The basis set convergence of both conventional and explicitly correlated methods has been investigated in detail, both with and without counterpoise corrections. For the MP2 and CCSD-MP2 contributions, rapid basis set convergence observed with explicitly correlated methods is compared to conventional methods. However, conventional, orbital-based calculations are preferred for the calculation of the (T) term, since it does not benefit from F12. CCSD(F12*) converges somewhat faster with the basis set than CCSD-F12b for the CCSD-MP2 term. The performance of various DFT methods is also evaluated for the BEGDB data set, and results show that Head-Gordon's ωB97X-V and ωB97M-V functionals outperform all other DFT functionals. Counterpoise-corrected DSD-PBEP86 and raw DSD-PBEPBE-NL also perform well and are close to MP2 results. In the WATER27 data set, the anionic (deprotonated) water clusters exhibit unacceptably slow basis set convergence with the regular cc-pVnZ-F12 basis sets, which have only diffuse s and p functions. To overcome this, we have constructed modified basis sets, denoted aug-cc-pVnZ-F12 or aVnZ-F12, which have been augmented with diffuse functions on the higher angular momenta. The calculated final dissociation energies of BEGDB and WATER27 data sets are available in the Supporting Information. Our best calculated dissociation energies can be reproduced through n-body expansion, provided one pushes to the basis set and electron correlation limit for the two-body term; for the three-body term, post-MP2 contributions (particularly CCSD-MP2) are important for capturing the three-body dispersion effects. Terms beyond four-body can be adequately captured at the MP2-F12 level.

Dynamics of pre- and post-insemination progesterone profiles and insemination outcomes determined by an in-line milk analysis system in primiparous and multiparous Canadian Holstein cows

The objective was to evaluate in-line milk progesterone (mP4) data to determine dynamics of pre- and post-insemination mP4 profiles and their associations with parity and outcomes of artificial insemination (AI) in Holstein cows. Milk progesterone (ng/mL) was quantified at pre-determined time points before and after AI through an automated in-line milk analysis system (Herd Navigator™, DeLaval International, Tumba, Sweden). Only AI (∼d0; n = 605) preceded by an mP4-decline (at least two samples of mP4 ≥5 ng/mL followed by at least one sample <5 ng/mL; d–2) were evaluated. Maximum mP4 attained between d–15 and d–2 (PrePeak), d–2, d5, d10, d14, maximum mP4 attained within 21d post-AI (PostPeak), and rate-of-daily-change between mP4 time points (ng/mL/d) were analyzed. Primiparous and multiparous cows were classified by AI outcomes based on post-AI mP4 profiles into three groups: (1) non-pregnant (OPEN; mP4-decline ≤ 30d post-AI), (2) presumed-pregnant (PREG; no mP4-decline until 55d post-AI), and (3) presumed-pregnancy loss (P-LOSS; mP4-decline between 31 and 55d post-AI). For profile comparisons, smoothed mP4 data were analyzed using mixed linear models. Primiparous cows had greater (P < 0.01) mP4 than multiparous cows at d5 (4.6 ± 0.2 vs. 2.8 ± 0.1), 10 (11.1 ± 0.4 vs. 7.6 ± 0.2), 14 (19.7 ± 0.4 vs. 16.1 ± 0.3) and PostPeak (23.5 ± 0.3 vs. 21.7 ± 0.2). The rate-of-daily-change was greater (P < 0.01) in primiparous than in multiparous cows from d–2 to 5 (+0.2 ± 0.03 vs. −0.1 ± 0.02) and from d5 to 10 (+1.2 ± 0.05 vs. +0.9 ± 0.03), but lesser (P < 0.01) from d14 to PostPeak (+0.9 ± 0.09 vs. +1.3 ± 0.06). In primiparous cows, mP4 in PREG was greater at d10 and PostPeak than OPEN (11.1 ± 0.5 and 24.2 ± 0.5 vs. 9.6 ± 0.4 and 22.3 ± 0.4, respectively, P < 0.04), but lesser at d5 than P-LOSS (4.4 ± 0.3 vs. 5.7 ± 0.4, P = 0.04). In multiparous cows, mP4 at d–2 was lesser in PREG than OPEN and P-LOSS (3.2 ± 0.1 vs. 3.4 ± 0.04 and 3.5 ± 0.1, respectively, P ≤ 0.03), but greater at d10, d14 and PostPeak in PREG than in OPEN (8.2 ± 0.4, 16.8 ± 0.5 and 22.7 ± 0.4 vs. 6.9 ± 0.3, 14.8 ± 0.3 and 19.7 ± 0.2, respectively, P < 0.01). Multiparous PREG cows had greater rate-of-daily-change in mP4 than OPEN cows from d5 to 10 and from d10 to 14 (+1.0 ± 0.06 and +2.2 ± 0.11 vs. +0.8 ± 0.04 and +1.9 ± 0.08, respectively, P < 0.03). Overall post-AI mP4 increased faster and were greater in primiparous than in multiparous cows. Based on in-line mP4 profiles, greater mP4 levels near time of AI (d–2 in multiparous and d5 in primiparous cows) and lesser mP4 beyond d10 were negatively associated with a successful pregnancy.

Role of the C(2)-H Hydrogen Bond Donor in Gas-Phase Microsolvation of Imidazole Derivatives with ROH (R = CH3, C2H5).

Although the hydrogen-bonding properties of the protic solvents are comparable, a comparison of the gas-phase structures of water with those of the alcohols reveals certain similarities as well as differences. In this work we report the microsolvated clusters of imidazole derivatives, Benzimidazole (BIM) and N-methylbenzimidazole (MBIM) by methanol (M) and ethanol (E) in supersonic jet using electronic and vibrational spectroscopy and compare them with their hydrated clusters. The cluster sizes up to 1:2 of BIM/MBIM with methanol and ethanol, and up to 1:3 in the case of MBIM-methanol were observed. Both the N-H···O and O-H···N bound structures were observed for the BIM-M1 and BIM-E1 complexes. The O-H···N bound structures of 1:1 complexes of MBIM were relatively more stable than their BIM counterparts by about ∼0.25 kcal mol-1. Three distinct conformations (anti, gauche, and gauche') were identified for the O-H···N bound complexes of BIM-E1 and MBIM-E1. IR spectroscopy of the doubly and triply solvated clusters (namely BIM-M2, MBIM-M2,3 and MBIM-E2) gives unequivocal proof of H-bonded bridges that originate from the N acceptor and terminate at the C(2)-H group, similar to the analogous water clusters. These studies confirm that the C(2)-H in imidazole plays an important role in its solvation, particularly in the case of polar solvents. Quantum chemical calculations performed at the DFT (B3LYP as well as dispersion-corrected functionals) and MP2 levels corroborate the experimental findings. Comparison of the QTAIM and NBO parameters for the pairwise solvent interactions in the clusters with those of methanol and ethanol homodimers reflects the co-operative nature of these H-bonding interactions.

How Does Mg2+ Modulate the RNA Folding Mechanism: A Case Study of the G:C W:W Trans Basepair

Reverse Watson-Crick G:C basepairs (G:C W:W Trans) occur frequently in different functional RNAs. This is one of the few basepairs whose gas-phase-optimized isolated geometry is inconsistent with the corresponding experimental geometry. Several earlier studies indicate that through post-transcriptional modification, direct protonation, or coordination with Mg2+, accumulation of positive charge near N7 of guanine can stabilize the experimental geometry. Interestingly, recent studies reveal significant variation in the position of putatively bound Mg2+. This, in conjunction with recently raised doubts regarding some of the Mg2+ assignments near the imino nitrogen of guanine, is suggestive of the existence of multiple Mg2+ binding modes for this basepair. Our detailed investigation of Mg2+-bound G:C W:W Trans pairs occurring in high-resolution RNA crystal structures shows that they are found in 14 different contexts, eight of which display Mg2+ binding at the Hoogsteen edge of guanine. Further examination of occurrences in these eight contexts led to the characterization of three different Mg2+ binding modes: 1) direct binding via N7 coordination, 2) direct binding via O6 coordination, and 3) binding via hydrogen-bonding interaction with the first-shell water molecules. In the crystal structures, the latter two modes are associated with a buckled and propeller-twisted geometry of the basepair. Interestingly, respective optimized geometries of these different Mg2+ binding modes (optimized using six different DFT functionals) are consistent with their corresponding experimental geometries. Subsequent interaction energy calculations at the MP2 level, and decomposition of its components, suggest that for G:C W:W Trans , Mg2+ binding can fine tune the basepair geometries without compromising with their stability. Our results, therefore, underline the importance of the mode of binding of Mg2+ ions in shaping RNA structure, folding and function.

How Hot are Your Ions Really? A Threshold Collision-Induced Dissociation Study of Substituted Benzylpyridinium "Thermometer" Ions.

The first absolute experimental bond dissociation energies (BDEs) for the main heterolytic bond cleavages of four benzylpyridinium "thermometer" ions are measured using threshold collision-induced dissociation in a guided ion beam tandem mass spectrometer. In this experiment, substituted benzylpyridinium ions are introduced into the apparatus using an electrospray ionization source, thermalized, and collided with Xe at varied kinetic energies to determine absolute cross-sections for these reactions. Various effects are accounted for, including kinetic shifts, multiple collisions, and internal and kinetic energy distributions. These experimentally measured 0K BDEs are compared with computationally predicted values at the B3LYP-GD3BJ, M06-GD3, and MP2(full) levels of theory with a 6-311+G(2d,2p) basis set using vibrational frequencies and geometries determined at the B3LYP/6-311+G(d,p) level. Additional dissociation pathways are observed for nitrobenzylpyridinium experimentally and investigated using these same levels of theory. Experimental BDEs are also compared against values in the literature at the AM1, HF, B3LYP, B3P86, and CCSD(T) levels of theory. Of the calculated values obtained in this work, the MP2(full) level of theory with counterpoise corrections best reproduces the experimental results, as do the similar literature CCSD(T) values. Lastly, the survival yield method is used to determine the characteristic temperature (Tchar) of the electrospray source prior to the thermalization region and to confirm efficient thermalization. Graphical Abstract ᅟ.

A computational study of the main degradation product of antitumor drug imexon

We theoretically study, from the thermodynamic point of view, the possibility of the two degradation pathways of antitumor drug of imexon. According to the theoretical results obtained at DFT level in both gas and aqueous phases, the two degradation pathways of imexon are characterized by negative reaction Gibbs free energies. It has been found that 4-imino-5-methylene-imidazolidin-2-one is the favored degradation product of imexon. Herein, the focus of this work has been on calculating the geometries, relative energies and electronic properties of all possible prototropic tautomers of 4-imino-5-methylene-imidazolidin-2-one using the DFT and MP2 levels. The bulk water environment has been simulated by continuum model using Solvation Model based on Density (SMD). The UV spectra of two dominant tautomers of 4-imino-5-methylene-imidazolidin-2-one are also predicted using time-dependent density functional theory and the results are compared with the available experimental data. The comparison of the simulated and the experimental absorption spectra has allowed us to accurately characterize the predominant tautomer of the degradation product in aqueous medium.

Conformational analysis of N-methylacetamide molecule in the ground and excited electronic states

Equilibrium geometry parameters, harmonic vibrational frequencies and barriers to conformational transitions and conformer energy differences for N-methylacetamide (CH3CONHCH3) molecule in S0, S1 and T1 electronic states were calculated in MP2 (S0) and CASPT2 (S1, T1) levels of theory. It is known that there are two planar (or effectively planar) conformers of N-methylacetamide in the ground electronic state. It is proved that the electronic excitation to aforementioned electronic states resulted in both CCON and HNCC fragments becoming pyramidal and rotated around «central» CN bond. One-, two- and three-dimensional potential energy surface (PES) sections by different large amplitude motions (LAM) coordinates were obtained by MP2 (S0) and CASPT2 (S1, T1) methods with cc-pVTZ basis set. On 2D PES sections by «central» torsion (CN) and one of the CCON or HNCC fragments’ inversion coordinates there are six different minima. Using PES sections as the potentials in model Hamiltonians, different anharmonic vibrational problems were solved and the frequencies of large amplitude vibrations were estimated variationally.

Interaction energies and structures of the $$\hbox {Li}^{+}{\cdot }(\hbox {CO})_{\hbox {n}}$$ Li + · ( CO ) n (n $$=$$ = 1–3) complexes

The bonding and structures of lithium ion carbonyl complexes, $$\hbox {Li}^{+}{\cdot }$$ (CO) $$_{1-3}$$ , were studied at the CCSD and MP2 levels of theories. A linear configuration is formed for the global minimum of the $$\hbox {Li}^{+}{\cdot }$$ CO and $$\hbox {Li}^{+}{\cdot }(\hbox {CO})_{2}$$ complexes with bond dissociation energies of 13.7 and 12.4 kcal $$\hbox {mol}^{-1}$$ , respectively. For the $$\hbox {Li}^{+}{\cdot }(\hbox {CO})_{3}$$ complex, a trigonal planar geometry is formed for the global minimum with a bond dissociation energy of 9.7 kcal $$\hbox {mol}^{-1}$$ . The computed sequential bond dissociation energies of $$\hbox {Li}^{+}{\cdot }(\hbox {CO})_{\mathrm{n}}$$ (n $$=$$ 1–3) complexes agreed with the experimental findings, in which the electrostatic energy plays an important role in the obtained trend. Lithium ion binds with carbon oxides and form three complexes of linear and trigonal planar geometries. These structures affect their sequential bond dissociation energies based on the strength of electrostatic energy contributions. These results match the experimental findings.

Calculation of Arrhenius Parameters by the Rrkm Method for the Carbon–Hydrogen Bond Fission Reaction of Fluorobenzene

The present study provides quantitative results for the rate of the unimolecular hydrogen–carbon bond fission reaction of fluorobenzene at elevated temperatures up to 2000 K. The potential energy surfaces for each C–H bond fission reaction (in the ortho, meta, and para sites) of fluorobenzene were investigated using ab initio calculations. The geometry and vibrational frequencies of the species involved in these reactions were optimised at the unrestricted MP2 level of theory, using the cc-pVDZ basis set. Since the C–H bond fission channel is a barrier-less reaction, we have used variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory to predict rate constants. The difficulty in the application of the RRKM method to molecules bigger than benzene is discussed and a method is offered to solve the problem. Using calculated rate constants at different temperatures, the activation energies and exponential factors were determined. The Arrhenius expressions for the C–H bond fission reactions of fluorobenzene at the ortho, meta and para sites were obtained as k( T) = 6.1 × 1016e−57328/ T, k( T) = 1.8 × 1017e−59080/ Tand k( T) = 1.3 × 1017e−59600/ Trespectively. Moreover, the effect of the fluorine atom including electron attraction and resonance with the benzene ring, on molecular rotation and the tunnelling effect on the rate expression have been discussed.

AuRnX and XAuRn (X=F ‐ I and OH)

AbstractSystematic theoretical investigations on the AuRnX and XAuRn (X=F‐I and OH) have been performed at B3LYP, MP2 and CCSD(T) theoretical levels. The linear structures, stretching frequencies and IR intensities were predicted. Analyses on dissociation energies and frontier orbitals show a decreased trend of stability down the periodic table for halogens. Natural bond orbital analyses show the σAu‐Rn and σAu‐X bonding occured between Au spd hybrids and Rn/X sp hybrids. The Au−Rn,Au−X and Rn−X interactions, characterized by negative energy densities and positive Laplacian values together with relative small electron densities at BCPs, can be described as interactions of moderate strength with partial covalence. Analyses on electron localization functions, electron density deformations and bonding orbital coefficients found an increased covalence trend down the periodic table for halogens.

A combined theoretical and experimental study of phenol-(acetylene)n (n ≤ 7) clusters.

Structures and vibrational spectra of phenol-(acetylene)n (Ph(Ac)n) clusters are studied by a combination of theoretical calculations and infrared (IR) spectroscopy. The molecular electrostatic potential features are utilized for generating trial geometries of the medium-sized Ph(Ac)n (n = 1, 2, 4, 6, and 7) clusters. These initial geometries are subjected to geometry optimization within the second-order Møller-Plesset (MP2) theory, employing correlation consistent aug-cc-pVDZ (aVDZ) basis set. Minimal nature of the reported structures is confirmed by doing vibrational frequency run at MP2/aVDZ level of theory using full calculations for n = 1 and 2 and employing grafting based molecular tailoring approach for the n = 4, 6, and 7. Several isomers for n ≥ 4 are found to lie in an energy window of 1 kcal mol-1 of each other. Considering the formidability of MP2 level investigation for the large number of isomers for n = 6 and 7, B97-D level theory is used for studying their energetics and IR spectra. It is seen that the number of energetically close isomers increases with increasing n. Moderately size-selected IR spectra of Ph(Ac)n (n = 4 and 7), prepared by a supersonic jet expansion, are measured for the acetylenic C-H and phenolic O-H stretch regions by infrared-ultraviolet (IR-UV) double resonance spectroscopy combined with time-of-flight mass spectrometry. Asymmetric line shape of the C-H stretch band and remarkable line broadening and weakening of the O-H stretch band are noteworthy features of the observed spectra. These findings of the experimental spectra are explained by the theoretical studies. The averaging of the vibrational spectrum of low-lying isomers of the Ph(Ac)n clusters lying in a narrow energy range is found to be responsible for the broadening and weakening of the O-H band.

Structural and spectroscopic properties of complexes formed between HNCS and SO2 in low temperature matrices

Matrix isolation FTIR spectroscopy has been combined with quantum chemical calculations in the aim to characterize complexes of isothiocyanic acid HNCS with SO2. The geometries of the 1:1, 1:2 and 2:1 complexes were optimized at the MP2 and DFT (B3LYPD3) levels of theory with the 6-311++G(3df,3pd) basis set. Five different HNCS⋯SO2 structures of the 1:1 stoichiometry were optimized. Three of them involve a weak NH⋯O hydrogen bond whereas two other geometries are stabilized by van der Waals interactions of various types. The HNCS/SO2/Ar spectra analysis evidences that at least one of the three hydrogen bonded structure is present after deposition of the matrices whereas the most stable van der Waals HNCS⋯SO2 structure as well as complexes of the 1:2 stoichiometry were detected upon annealing.

Electronic and vibrational second hyperpolarizabilities of (MgO)n clusters.

In this work, we report results for the static second hyperpolarizability of magnesium oxide clusters including electronic and vibrational contributions. The comparison between second-order Møller-Plesset (MP2) perturbation theory and coupled cluster results to the electronic contribution points out that MP2 is a suitable method to compute this property. When computed at the MP2 level, the electronic contribution per atom converges to approximately 5000 a.u. Vibrational corrections were computed at the MP2 level through the perturbation theoretical method of Bishop and Kirtman. Results obtained showed that the term [α2]0,0 represents around 20% of the electronic counterpart while the term [μβ]0,0 is comparable to it. Modes that contribute significantly to [α2]0,0 are those in which all or part of the bond lengths simultaneously increase and decrease, leading to large polarizability derivatives. In turn, modes that provide relevant contributions to [μβ]0,0 are those in which oxygen anions move in opposite directions to the magnesium cations yielding large derivatives of the dipole moment and first hyperpolarizability.