Infrared absorption by collisional CH4+X pairs, with X=He, H2, or N2

Existing measurements of the collision-induced rototranslational absorption spectra of gaseous mixtures of methane with helium, hydrogen, or nitrogen are compared to theoretical calculations, based on refined multipole-induced and dispersion force-induced dipole moments of the interacting molecular pairs CH4-He, CH4-H2, and CH4-N2. In each case the measured absorption exceeds the calculations substantially at most frequencies. We present the excess absorption spectra, that is the difference of the measured and the calculated profiles, of these supramolecular CH4-X systems at various gas temperatures. The excess absorption spectra of CH4-X pairs differ significantly for each choice of the collision partner X, but show common features (spectral intensities and shape) at frequencies from roughly 200 to 500 cm(-1). These excess spectra seem to defy modeling in terms of ad hoc exchange force-induced dipole components attempted earlier. We suggest that besides the dipole components induced by polarization in the electric molecular multipole fields and their gradients, and by exchange and dispersion forces, other dipole induction mechanisms exist in CH4-X complexes that presumably are related to collisional distortion of the CH4 molecular frame.     

Characterization of the interface dipole at the paraphenylenediamine-nickel interface: a joint theoretical and experimental study

In organic-based (opto)electronic devices, charge injection into conjugated materials is governed to a large extent by the metal-organic interface dipole. Controlling the injection of charges requires a better understanding of the fundamental origin of the interface dipole. In this context, photoelectron spectroscopies and density functional theory calculations are used to investigate the interaction between para-phenylenediamine (PPDA), an electron donor, and a polycrystalline nickel surface. The interface dipole formed upon chemisorption of one PPDA monolayer strongly modifies the work function of the nickel surface from 5.10 to 3.55 eV. The work function decrease of 1.55 eV is explained by the electron-donor character of PPDA and the modification of the electronic density at the metal surface. PPDA monolayers are composed of tilted molecules interacting via the nitrogen lone-pair and PPDA molecules chemisorbed parallel to the surface via their pi-electron density. Annealing the monolayer leads to dehydrogenation of PPDA activated by the nickel surface, as found for other amines.     

Structural and spectroscopic studies on some chloropyrimidine derivatives by experimental and quantum chemical methods

Ab initio quantum chemical and experimental spectroscopic studies in the infrared (4000-60 cm(-1)) and UV spectral regions are being reported on 4-chloro-2,6-dimethylsulfanyl pyrimidine-5-carbonitrile and 4-chloro-2-methylsulfanyl-6-(2-thienyl) pyrimidine-5-carbonitrile. Optimized geometries, electronic charge distribution, dipole moments and three-dimensional molecular electrostatic potential surfaces have been obtained. These have been used to understand the structure and spectral characteristics of the two compounds. A complete assignment of vibrational spectra on the basis of DFT/6-311G** and electronic spectra on the basis of TD-DFT/6-31+G* 5D calculations have been attempted for the two molecules.     

A theoretical study of the chiroptical properties of molecules with isotopically engendered chirality

There has been a considerable interest in the chiroptical properties of molecules whose chirality is exclusively due to an isotopic substitution and numerous examples for the electronic circular dichroism (CD) spectra of isotopically chiral systems have been reported in literature. Four different explanations have been proposed for the mechanism as to how the isotopic substitution induces a chiral perturbation of the otherwise achiral electronic wave function; however, up to now no conclusive answer has been given about the dominating effect responsible for the experimental observations. In this study we will present, for the first time, fully quantum-mechanical calculations of the CD spectra of three different molecular systems with isotopically engendered chirality. As examples, we consider the spectra of organic molecules with ketone and alpha-diketone carbonyl and diene chromophores. The effect of vibronic couplings for the reorientation of the electric and magnetic transition dipole moments is taken into account within the Herzberg-Teller approximation. The ground and excited state geometries and vibrational normal modes are obtained with (time-dependent) density functional theory [(TD)DFT], while the vibronic coupling effects are calculated at the TDDFT and density functional theory/multireference configuration interaction (DFT/MRCI) levels of theory. Generally, the band shapes of the experimental CD spectra are reproduced very well, and also the absolute CD intensities from the simulations are of the right order of magnitude. The sign and the intensity of the CD band are determined by a delicate balance of the contributions of a large number of individual vibronic transitions, and it is found that the vibrational normal modes with a large displacement are dominant. The separation of the calculated CD spectrum into the different contributions due to the overlap of the in-plane and out-of-plane components (regarding the symmetry plane of the unsubstituted molecule) of the electric and magnetic transition dipole moments yields information about the influence of the vibronic coupling effects for the reorientation of the corresponding transition dipole moments. In conclusion, the calculations clearly show that vibronic effects are responsible or at least dominant for the chiroptical properties of isotopically chiral organic molecules.     

Spectroscopic observation of nitrous oxide pentamers

Two new infrared bands in the ν(1) fundamental region of N(2)O are observed in a supersonic jet expansion and assigned to nitrous oxide pentamers. Each band is measured using both (14)N(2)(16)O and (15)N(2)(16)O. Although they are similar in appearance, the bands have slightly different lower state rotational parameters, and are thus assigned to distinct structural isomers of the pentamer. Cluster calculations using two N(2)O intermolecular potentials give results in good agreement with the observed spectra, and indicate that the two isomers probably have the same basic structure (which is unsymmetrical), but differ in the alignment (N-N-O or O-N-N) of one or two of the constituent monomers. Calculations using a resonant dipole interaction model also support the proposed assignment and structure. These are the first reported high-resolution spectra for N(2)O pentamers.     

OSCILLATOR AND DIPOLE STRENGTHS FOR CHLOROPHYLL AND RELATED MOLECULES*

Abstract. –Previous calculations of the oscillator and dipole strengths for chlorophyll and related molecules are critically reviewed and, in some cases, important information that was not included in the original published reports is given. Oscillator and dipole strengths are reported for the Q _y (0,0) vibronic transition of chlorophyll a , bacteriochlorophyll a , pyrochlorophyll a , pheophytin a , and pyrobacteriochlorophyll a. A medium correction has been explicitly included in the calculation of chlorophyll oscillator and dipole strengths. The proper formulas are summarized for the calculation of oscillator and dipole strengths from visible absorption spectra. Three criteria for the full specification of an oscillator strength or dipole strength calculation are recommended; (i) the formula used must be given explicitly, (ii) it must be made clear whether or not a medium correction was applied (and if a medium correction was applied, then it should be given explicitly), and (iii) the limits of integration over v must be specified.     

Dipole moments studies of fluorenone and 4-hydroxyfluorenone

Electronic absorption, excitation and fluorescence spectra of fluorenone and 4-hydroxyfluorenone were recorded at room temperature in several aprotic solvent of varying polarities. The ground (mu(g)) and excited (mu(e)) state dipole moments of both molecules were estimated from solvatochromic shifts of absorption and fluorescence spectra as a function of the dielectric constant (epsilon) and refractive index (n). These experimental results were completed with theoretical results of quantum chemical calculations (AM1). The experimental and theoretical dipole moments in the ground state were compared. It was determined that dipole moments of excited state were higher than those of the ground state for both molecules.     

Two-dimensional Raman spectra of atomic solids and liquids

We calculate third- and fifth-order Raman spectra of simple atoms interacting through a soft-core potential by means of molecular-dynamics (MD) simulations. The total polarizability of molecules is treated by the dipole-induced dipole model. Two- and three-body correlation functions of the polarizability at various temperatures are evaluated from equilibrium MD simulations based on a stability matrix formulation. To analyze the processes involved in the spectroscopic measurements, we divide the fifth-order response functions into symmetric and antisymmetric integrated response functions; the symmetric one is written as a simple three-body correlation function, while the antisymmetric one depends on a stability matrix. This analysis leads to a better understanding of the time scales and molecular motions that govern the two-dimensional (2D) signal. The 2D Raman spectra show novel differences between the solid and liquid phases, which are associated with the decay rates of coherent motions. On the other hand, these differences are not observed in the linear Raman spectra.     

Normal coordinate analyses and barrier to internal rotation of nitroso- and nitroazides

The conformational and structural stability of nitrosoazide NNN-N=O and nitroazide NNN-NO2 were investigated by DFT-B3LYP and ab initio MP2 calculations with 6-311++G** basis set. From the calculations, nitrosoazide was predicted to exist predominantly in the planar trans (NNN and N=O groups are trans to each other) structure with high trans-cis rotational barrier of about 11 kcal mol-1 as a result of pronounced conjugation between the azide group and the N=O bond. The NO2 rotational barrier in nitroazide was predicted from the symmetric potential function to be of about 7 kcal mol-1. The vibrational frequencies were calculated at the DFT-B3LYP level and the infrared and Raman spectra of the cis-trans mixture were plotted. Complete vibrational assignments were made on the basis of normal coordinate calculations for the stable conformers of both molecules. For nitrosoazide, the calculated wavenumbers were compared to the corresponding experimental values obtained from early reported Raman spectrum of the molecule.     

Reduced-size polarized basis sets for calculations of molecular electric properties. I. The basis set generation

Following the recent studies of basis sets explicitly dependent on oscillatory external electric field we have investigated the possibility of some further truncation of the so-called polarized basis sets without any major deterioration of the computed data for molecular dipole moments, dipole polarizabilities, and related electric properties of molecules. It has been found that basis sets of contracted Gaussian functions of the form [3s1p] for H and [4s3p1d] for the first-row atoms can satisfy this requirement with particular choice of contractions in their polarization part. With m denoting the number of primitive GTOs in the contracted polarization function, the basis sets devised in this article will be referred to as the ZmPol sets. In comparison with earlier, medium-size polarized basis sets (PolX), these new ZmPol basis sets are reduced by 2/3 in their size and lead to the order of magnitude computing time savings for large molecules. Simultaneously, the dipole moment and polarizability data remain at almost the same level of accuracy as in the case of the PolX sets. Among a variety of possible applications in computational chemistry, the ZmPolX are also to be used for calculations of frequencies and intensities in the Raman spectra of large organic molecules (see Part II, this issue).     

Mixed quantum-classical description of spectroscopy of dissipative systems

Mixed quantum-classical statistical mechanics is employed to calculate dipole moment correlation function and linear absorption spectra. A quantum two-level subsystem interacting with quantum vibrations (primary oscillators) which in turn are coupled to a classical bath composed of infinite set of harmonic oscillators is used as a dissipative system. Starting with mixed quantum-classical Liouville equation for the evaluation of the mixed quantum-classical dipole moment correlation function and using coherent states and the inverse of Baker-Campbell-Hausdorf formula to evaluate the trace over the primary oscillators, whereby, a closed analytical expression for the electronic dipole moment correlation function is obtained. Illustrations of several absorption spectra at different temperatures are provided. An approximate optical four-point correlation is obtained in the high temperature limit. A strategy for deriving an exact optical four-point correlation is suggested.     

Infrared matrix isolation and ab initio quantum mechanical studies of purine and adenine

Results are presented from ab initio SCF (3-21G) calculations for the geometries of the N(9)H and N(7)H tautomers of purine and adenine and vibrational spectra (wavenumbers and intensities) of the N(9)H forms. All these results are compared with available geometries from crystallographic studies and with reported infrared spectra of the molecules isolated in inert gas low-temperature matrices. The N(9)⇌N(7)H tautomerism of the molecules in question is also briefly discussed.     

氨基酸分子的光学活性和频效应

用带限制场Hartree-Fock (RHF/6-311++G^**)从头算的方法进行分子构型优化, 单激发态组态相互作用(CIS/6-311++G^**)计算各种需要的相关数据; 应用态求和理论, 以长度偶极矩和速度偶极矩两种形式模拟了一系列氨基酸离子的光学活性和频效应(OA-SFG)光谱. 结果表明速度偶极矩计算得到的一系列氨基酸离子的和频效应光谱强度顺序能很好地与实验相吻合, 而长度偶极矩形式计算得到的结果与实验不吻合. 引起这两种形式和频效应光谱理论计算结果差别的原因是由于长度偶极矩形式具有原点敏感性, 而速度偶极矩没有. 速度偶极矩方法更适用于分子光学活性和频效应的模拟.     

On the vibrational spectra and structural parameters of methyl, silyl, and germyl azide from theoretical predictions and experimental data

The infrared and Raman spectra of methyl, silyl, and germyl azide (XN3 where X=CH3, SiH3 and GeH3) have been predicted from ab initio calculations with full electron correlation by second order perturbation theory (MP2) and hybrid density function theory (DFT) by the B3LYP method with a variety of basis sets. These predicted data are compared to previously reported experimental data and complete vibrational assignments are provided for all three molecules. It is shown that several of the assignments recently proposed [J. Mol. Struct. (Theochem.) 434 (1998) 1] for methyl azide are not correct. Structural parameters for CH3N3 and GeH3N3 have been obtained by combining the previously reported microwave rotational constants with the ab initio MP2/6-311+G(d,p) predicted values. These "adjusted r0" parameters have very small uncertainties of +/-0.003 A for the XH distances and a maximum of +/-0.005 A for the heavy atom distances and +/-0.5 degrees for the angles. The predicted distance for the terminal NN bond which is nearly a triple bond is much better predicted by the B3LYP calculations, whereas the fundamental frequencies are better predicted by the scaled ab initio calculations. The results are discussed and compared to those obtained for some similar molecules.     

Atoms-in-molecules calculation of dipole moments of the lowest Σ and Π states of LiH

AIM calculations of dipole and transition moment functions are reported for the lowest Σ and Π states of LiH using a relatively small basis. The functions obtained agree quite satisfactorily with the best ab initio calculations. The results seem to indicate that the AIM approach might become useful in producing missing input data to the DIM calculations of dipole and transition moment surfaces for molecular collision systems interacting with a laser field.     

Unusually Strong Dipole–Dipole and Dipole–Quadrupole Interactions in a Nanoporous Solid. Crystal Structure and Related Properties of (VO)3[M(CN)6]2·nH2O (M = Fe,Co)

In porous Prussian blue (PB) analogues, the partially naked central metal atoms found at the cavities surface are responsible for many of their physical properties, among them the adsorption potentials. In the as‐synthesized PB analogues, such metal sites stabilize water molecules inside the cavity through coordination bond formation. The filling of the cavity volume is completed with water molecules linked to the coordinated ones through hydrogen bonds formation. Vanadyl‐based PB analogue shows quite different features. The metal(V) at the cavities surface has saturated its coordination sphere with the O atom of the vanadyl ion (V=O). In this material, the V=O group preserves enough strong dipole moment to stabilize adsorbed species at the cavity through dipole–dipole and dipole–quadrupole interactions. This contribution reports the preparation, crystal structure and properties for (VO)₃[M(CN)₆]₂ · nH₂O (M = Fe, Co). According to the refined crystal structure, IR spectra and TG data, six water molecules remain stabilized inside the cavities through a strong dipole–dipole coupling with the vanadyl group. The cavity contains additional water molecules interacting through hydrogen bond bridges with the water molecules coupled to the V=O group. The vanadyl ion is free of hydrogen bonding interactions with the water molecules. The recorded adsorption isotherms for N₂, CO₂ and H₂, three molecules with only quadrupole moment, reveal presence of relative strong adsorption forces due to dipole‐quadrupole interactions.     

Theoretical studies on the ionization potential of interacting atoms at large separations

The theory of the one-particle Green's function is applied to calculations of the ionization potential of interacting atoms which are at large separations. Equations for the ionization potential involve terms which relate to Van der Waals interactions between separated atoms and long-range interactions between an atom and an ion. Numerical calculations of the ionization potential of two hydrogen atoms and two helium atoms at large separations are performed. Applications to the ionization potentials of weakly-interacting Van der Waals molecules (NeAr, NeKr, NeXe) are also reported.