Comparison of some vibrational features of FArH...Rg and FHc...Rg complexes (Rg=He, Ne, Ar, Kr)

The shift in the harmonic vibrational frequency of the ArH stretch of FArH on formation of the linear FArH...Rg complexes (Rg=He, Ne, Ar, Kr), and of the FH stretch on formation of the linear FH...Rg complexes, has been determined by ab initio computations. These shifts are in agreement with predictions from a model based on perturbation theory and involving the first and second derivatives of the interaction energy with respect to displacement of the ArH (FH) bond length from its equilibrium value in the monomer. In the FH...Rg dimers, small blue shifts were obtained for the He and Ne complexes and red shifts for those with Ar and Kr. In the FArH...Rg dimers, blue shifts were obtained for all four complexes. These vibrational characteristics are rationalized by considering the balance between the interaction energy derivatives obtained from the perturbative model. The bond length changes on complexation are also well predicted by the model. Our computations were restricted to the linear geometry since the objective was to investigate the validity of the perturbation model and to illuminate the causes of the red and blue shifts.     

On the correlation between bond-length change and vibrational frequency shift in halogen-bonded complexes

The C-Hal (Hal = Cl, Br, or I) bond-length change and the corresponding vibrational frequency shift of the C-Hal stretch upon the C-Hal···Y (Y is the electron donor) halogen bond formation have been determined by using density functional theory computations. Plots of the C-Hal bond-length change versus the corresponding vibrational frequency shift of the C-Hal stretch all give straight lines. The coefficients of determination range from 0.94366 to 0.99219, showing that the correlation between the C-Hal bond-length change and the corresponding frequency shift is very good in the halogen-bonded complexes. The possible effects of vibrational coupling, computational method, and anharmonicity on the bond-length change-frequency shift correlation are discussed in detail.     

A comparative computational study of FKrCCH...Y, FCCKrH...Y, and FCCH...Y complexes (Y = BF, CO, N2, OH2, OH(CH3), O(CH3)2)

The structural and spectroscopic changes in complexes of FCCKrH...Y and FKrCCH...Y (Y = BF, CO, N(2), OH(2), OH(CH(3)), O(CH(3))(2)) were computed at the MP2∕6-31++G(d,p) level of theory and compared with the corresponding properties for FCCH...Y. The computed bond length changes and frequency shifts on complexation were rationalized by comparing with a perturbation model, which gives quantitative agreement with the standard ab initio results. A recently proposed model also gives a reasonable qualitative account of the observed trends in these complexes.     

A comparative study of some red- and blue-shifted linear H-bonded complexes of N2

Bond length changes, harmonic vibrational frequency shifts, and changes in the proton magnetic shielding of HX and HKrX (X = F, Cl) on complexation with N2 to form the linear red-shifted N2 ... HX and linear blue-shifted N2 ... HKrX complexes were determined by ab initio computations, with and without counterpoise correction, at the SCF and MP2(full) levels of theory using a 6-311++G(2d,2p) basis set. The MP2 computations agree with predictions from a perturbation theory model involving the first and second derivatives of the interaction energy with respect to displacement of the H--X and H--Kr bond lengths from their equilibrium values in the isolated monomers. The theoretical results agree qualitatively with the experimentally observed frequency shifts, with near quantitative agreement for N2 ... HKrCl. The characteristic downfield shift of the isotropic proton magnetic resonance in the red-shifted complexes was obtained, but for the blue-shifted complexes, the proton NMR shifts to higher fields.     

Experimental and computational study of HXeY...HX complexes (X, Y = Cl and Br): an example of exceptionally large complexation effect

The complexes of xenon hydrides HXeY (Y = Cl and Br) with hydrogen halides HX (X = Cl and Br) have been studied both computationally and experimentally in a xenon matrix. The experiments revealed three new complexes: HXeBr...HBr, HXeBr...HCl, and HXeCl...HCl. The experimental assignments were done on the basis of the strong H-Xe stretching absorption of HXeY (Y = Cl and Br) molecules and supported by theoretical results. We experimentally obtained monomer-to-complex blue-shifts of this vibrational mode for all the studied systems (up to approximately 150 cm (-1)). The electronic structure calculations revealed three local structures for each HNgY...HX complexes and their computed interaction energies varied between -460 and -2800 cm (-1). The computational estimates of the vibrational shifts were in agreement with the experimental values. We also found possible experimental absorption belonging to HXeBr...(HBr) 2 trimer and its vibrational shift (+245 cm (-1)) is similar to the computational estimate of a cyclic ternary complex (+252 cm (-1)).     

Adsorption of HF and HCl on the beta-AlF3 (100) surface

The current study employs hybrid-exchange density functional theory to investigate the adsorption of HF and HCl to under-coordinated Al ions on the beta-AlF(3) (100) surface. It is shown that the geometries of the adsorbates are strongly dependent on coverage. Furthermore, the adsorption of HCl leads to a number of distinct structures that have very similar energies. It is proposed that this result may explain the high catalytic activity of aluminium fluoride and aluminium chloro-fluoride surfaces towards chlorine-fluorine exchange reactions. The stretching and bending frequencies of the H-F and H-Cl bonds at half and full monolayer coverage are also calculated and the vibrational spectrum is found to be strongly dependent on the adsorption site and the coverage. The vibrational frequency shifts provide, therefore, a mechanism for experimentally characterising these surfaces.     

Matrix isolation infrared spectroscopic studies and density functional theory calculations of the MNN, (MN)2 (M = Y and La), and Y3NN molecules

The reactions of yttrium and lanthanum with dinitrogen were reinvestigated. Laser-ablated yttrium and lanthanum atoms were co-deposited at 4 K with dinitrogen in excess argon, and the low-temperature reactions of Y and La with N2 in solid argon were studied using infrared spectroscopy. The reaction products YNN, (YN)2, LaNN, and (LaN)2 were formed in the present experiments and characterized on the basis of 14N/15N isotopic shifts, mixed isotope splitting patterns, stepwise annealing, change of reagent concentration and laser energy, and comparison with theoretical predictions. Some assignments were made based on a previous report. Density functional theory calculations were performed on these systems to identify possible reaction products. The agreement between experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts of the MNN and (MN)2 (M = Y and La) molecules supports the identification of these molecules from the matrix infrared spectra. Plausible reaction mechanisms were proposed for the formation of these molecules along with tentative identification of the Y3NN molecule.     

CO-NO and CO-O(2) interactions on Cu(100) between 25 and 200 K studied with infrared reflection absorption spectroscopy

Infrared reflection absorption spectroscopy (IRAS) has been used to study CO-NO and CO-O(2) interactions on Cu(100) between 25 and 200 K. A strong repulsive interaction between CO and NO on Cu(100) at 25 K causes tilting of the CO molecules away from the surface normal and a blue-shift of the CO vibrational frequency. Upon warming and decomposition of the NO, the CO molecules return to a bonding position normal to the surface plane. The vibrational frequency of CO blue-shifts from 2087 to 2136 cm(-1) upon coadsorption with N and O atoms formed from NO decomposition. On the other hand, the interaction of CO with O(2) at 27 K on Cu(100) in the submonolayer regime induces a red-shift of the CO vibrational frequency. Atomic oxygen, formed on Cu(100) by dissociation of O(2) at 95 K, induces a blue-shift of the vibrational frequency of coadsorbed CO to 2116 cm(-1). The CO vibrational frequency shifts to 2091 cm(-1) when the surface is annealed to 140 K, implying a change in the adsorption geometry of the oxygen atoms on Cu(100).     

Relationships between NMR shifts and interaction energies in biphenyls,alkanes, aza-alkanes,and oxa-alkanes with X─H…H─Y and X─H…Z (X,Y = C or N; Z = N or O) hydrogen bonding

Hydrogen–hydrogen C─H^…H─C bonding between the bay-area hydrogens in biphenyls, and more generally in congested alkanes, very strained polycyclic alkanes, and cis-2-butene, has been investigated by calculation of proton nuclear magnetic resonance (NMR) shifts and atom–atom interaction energies. Computed NMR shifts for all protons in the biphenyl derivatives correlate very well with experimental data, with zero intercept, unit slope, and a root mean square deviation of 0.06 ppm. For some congested alkanes, there is generally good agreement between computed values for a selected conformer and the experimental data, when it is available. In both cases, the shift of a given proton or pair of protons tends to increase with the corresponding interaction energy. Computed NMR shift differences for methylene protons in polycyclic alkanes, where one is involved in a very short contact (“in”) and the other is not (“out”), show a rough correlation with the corresponding C─H^…H─C exchange energies. The “in” and “in,in” isomers of selected aza- and diaza-cycloalkanes, respectively, are X─H^…H─N hydrogen bonded, whereas the “out” and “in,out” isomers display X─H^…N hydrogen bonds (X = C or N). Oxa-alkanes and the “in” isomers of aza–oxa-alkanes are X─H^…O hydrogen bonded. There is a very good general correlation, including both N─H^…H─Y (Y = C or N) and N─H^…Z (Z = N or O) interactions, for NH proton shifts against the exchange energy. For “in” CH protons, the data for the different C─H^…H─Y and C─H^…Z interactions are much more dispersed and the overall shift/exchange energy correlation is less satisfactory.     

氢键复合物中键长变化与振动频率移动相关性重访(英文)

X―H···Y(Y为电子供体)型氢键形成时,X―H键长伸长或缩短与相应的X―H伸缩振动频率红移或蓝移存在较强的相关性,这也是氢键光谱检测和研究的基础.但是,最近的理论研究却推翻了这一观点,认为X―H键长变化和相应的X―H伸缩振动频率移动在有些氢键体系中并不存在相关性(McDowell,S.A.C.;Buckingham,A.D.J.Am.Chem.Soc.2005,127,15515.).本文中,我们采用更为可信的计算方法,对这一问题进行再研究.结果表明是错误的计算方法导致了McDowell和Buckingham得出错误的结论.在McDowell和Buckingham所研究的氢键体系中,X―H键长变化和相应的X―H伸缩振动频率移动仍存在较强的相关性.     

Reactions of laser-ablated La and Y atoms with CO: matrix infrared spectra and DFT calculations of the M(CO)x and MCO+ (M = La, Y; x = 1-4) molecules

Reactions of laser-ablated lanthanum and yttrium atoms with carbon monoxide molecules in solid neon have been investigated using matrix-isolation infrared spectroscopy. The M(CO)x and MCO+ (M = La, Y; x = 1-4) molecules have been formed and identified on the basis of isotopic shifts, mixed isotopic splitting patterns, and CCl4-doping experiments. Density functional theory calculations have been performed on these lanthanum and yttrium carbonyls. The agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts substantiates the identification of these carbonyls from the matrix infrared spectrum. The present study reveals that the C-O stretching vibrational frequencies of MCO+ decrease from Sc to La, which indicates an increasing in metal d orbital --> CO pi* back-donation in this series.     

Vibrational solvatochromism and electrochromism of infrared probe molecules containing C≡O, C≡N, C=O, or C-F vibrational chromophore

Solvatochromic vibrational frequency shifts of a few different infrared (IR) probe molecules have been studied by carrying out quantum chemistry calculations for a number of their water clusters. We are particularly focused on the vibrational solvatochromic and electrochromic effects on the CO, CN, and CF stretch modes in carbon monoxide, acetone, 4-cyanopyridine, p-tolunitrile, fluorobenzene, and 3-fluoropyridine. Using multiple interaction site antenna model, we show that their solvatochromic vibrational frequency shifts can be successfully described by considering spatially nonuniform electrostatic potential generated by the surrounding water molecules. It turns out that the CO and CF stretch mode frequencies are approximately proportional to the solvent electric field projected onto the bond axes, whereas the vibrational frequencies of the nitrile stretch mode in 4-cyanopyridine and p-tolunitrile are not. Consequently, it is confirmed that the vibrational Stark tuning rates of the CO and CF stretching modes can be directly used to describe their solvatochromic frequency shifts in condensed phases. However, the nitrile stretch mode frequency shift induced by solvent electrostatic potential appears to be more complicated than its electrochromic phenomenon. To examine the validity of the distributed interaction site model for solvatochromic frequency shifts of these vibrational chromophores, we thus calculated the vibrational Stark tuning rates of the CO, CN, and CF stretch modes and found that they are in good agreement with the experimental results found in literatures. This confirms that a collection of properly chosen distributed interaction sites can be an excellent electric antenna sensing local electrostatics that affects on vibrational frequencies of IR probe modes.     

An electrostatic model for the frequency shifts in the carbonmonoxy stretching band of myoglobin: correlation of hydrogen bonding and the stark tuning rate

The effect of internal and applied external electric fields on the vibrational stretching frequency for bound CO (nu(CO)) in myoglobin mutants was studied using density functional theory. Geometry optimization and frequency calculations were carried out for an imidazole-iron-porphine-carbonmonoxy adduct with various small molecule hydrogen-bonding groups. Over 70 vibrational frequency calculations of different model geometries and hydrogen-bonding groups were compared to derive overall trends in the C-O stretching frequency (nu(CO)) in terms of the C-O bond length and Mulliken charge. Simple linear functions were derived to predict the Stark tuning rate using an approach analogous to the vibronic theory of activation.(1) Potential energy calculations show that the strongest interaction occurs for C-H or N-H hydrogen bonding nearly perpendicular to the Fe-C-O bond axis. The calculated frequencies are compared to the structural data available from 18 myoglobin crystal structures, supporting the hypothesis that the vast majority of hydrogen-bonding interactions with CO occur from the side, rather than the end, of the bound CO ligand. The nu(CO) frequency shifts agree well with experimental frequency shifts for multiple bands, known as A states, and site-directed mutations in the distal pocket of myoglobin. The model calculations quantitatively explain electrostatic effects in terms of specific hydrogen-bonding interactions with bound CO in heme proteins.     

C_4H_4Y(Y=O,S,Se)与BX_3(X=H,F,Cl)作用的计算化学研究

采用量子化学的密度泛函B3LYP和二阶微扰MP2(full)方法对C4H4Y(Y=O,S,Se)与BX3(X=H,F,Cl)形成的电子授受型复合物进行了研究,所得18个复合物的构型包括BX3位于C=C双键上方的π-p作用型和B与O,S,Se直接作用的n-p作用型.体系C4H4Y-BH3以n-p作用型较为稳定,体系C4H4Y-BF3,C4H4Y-BCl3的π-p和n-p作用型复合物稳定性相当.对各复合物的几何构型、振动频率和自然键轨道分析表明,复合物的形成过程中均存在几何构型的改变、电荷的转移和振动频率的变化,它们的变化规律与复合物稳定性的变化规律基本一致,即按H,F,Cl的顺序依次降低.     

Repulsion forces in vibrational spectroscopy—I. Spectral shifts in vibrational spectra of condensed media caused by repulsion forces

Repulsional contributions into spectral shifts have been expressed in terms of derivatives of atom displacements with respect to vibrational normal coordinates. Attraction and repulsion have been shown to give a comparable contribution to the overall shift. Working ability of the formulae obtained for spectral shift calculations for molecular (HCl solution in CCl₄) and ionic (molten lithium and rubidium nitrates) liquids is demonstrated.     

Solute-solvent interaction in liquids. I. Long-range forces and vibrational solvent shifts

The model of solute— solvent interaction based on dipole-induced-dipole forces (Kirkwood-Bauer-Magat) has been generalized, yielding an expression for the energy as a function of solute position and orientalion within a spherical cavity in a dielectric medium. An analogous relation has been derived for the dispersion energy. Barriers to rotation of the solute molecule and shifts in its vibrational frequency are calculated as functions of cavity radius and eccentricity for the case of dilute solutions of HCl in CCl₄. It is found that the effect of dispersion forces on the vibrational frequency of HCl is two-to-three times more important than the traditional dipole-induced-dipole contribution.     

Origin of the X-Hal (Hal = Cl, Br) bond-length change in the halogen-bonded complexes

The origin of the X-Hal bond-length change in the halogen bond of the X-Hal...Y type has been investigated at the MP2(full)/6-311++G(d,p) level of theory using a natural bond orbital analysis, atoms in molecules procedure, and electrostatic potential fitting methods. Our results have clearly shown that various theories explaining the nature of the hydrogen bond cannot be applied to explain the origin of the X-Hal bond-length change in the halogen bond. We provide a new explanation for this change. The elongation of the X-Hal bond length is caused by the electron-density transfer to the X-Hal sigma* antibonding orbital. For the blue-shifting halogen bond, the electron-density transfer to the X-Hal sigma* antibonding orbital is only of minor importance; it is the electrostatic attractive interaction that causes the X-Hal bond contraction.     

A new perspective in the Lewis acid catalyzed ring opening of epoxides. Theoretical study of some complexes of methanol, acetic acid, dimethyl ether, diethyl ether, and ethylene oxide with boron trifluoride

Several 1:1, 1:2, and 2:2 complexes between BF3 and CH3OH (Met), CH3COOH (AcA), (CH3)2O (DME), (CH3CH2)2O (DEE), and (CH2)2O (EOX) have been studied using ab initio (MP2) and density functional theory (DFT) (PBE, B3LYP) methods and the 6-311++G(3df,2pd) basis set. Geometrical structures and vibrational frequencies are reported, in most cases, for the first time. A detailed comparison of the vibrational frequencies for the O...BF3 vibrational modes, as well as for the nu(OH) band in the methanol and acetic acid complexes with BF3, is performed, and the theoretical frequency shifts are compared with the available experimental information. Thermochemical properties are calculated by employing counterpoise correction to alleviate the basis set superposition error. The DFT enthalpy of complexation of the 1:1 complexes results in the order of stability (AcA)2>AcA:BF3>DEE:BF3>DME:BF3>Met:BF3>EOX:BF3>(Met)2; in contrast, MP2 shows the noticeable difference that the AcA:BF3 complex is much less stable (similar to Met:BF3). The order of stability shows that, even though acetic acid prefers dimerization to complexation with BF3, the case is exactly the opposite for methanol. In both cases, the interaction of BF3 with the dimer gives rise to very stable trimers. However, in contrast to the interaction of BF3 with the methanol dimer being stronger than that with the monomer, the interaction of BF3 with the acetic acid dimer is weaker than that with the monomer. The relative strength of the complexes, discussed in the context of BF3-catalyzed ring opening of epoxides, suggests that the effect of the catalyst in a nonprotogenic solvent should be more properly ascribed to activation of the nucleophile instead of activation of the epoxide.     

The geometry and vibrational frequency shift of CO molecules in an argon matrix studied by force-field calculations

In this paper, the CO·Ar interaction potential including pairwise and three-body contributions which reproduces the experimental CO vibrational frequency shift, Δω = ?4.7 cm^?1 in a static force-field approximation, is presented. The geometry of the COAr solid system is described in detail. The frequency shift is obtained when the CO molecule is oriented along the (0, 0, 1) crystal axis, with its center of mass shifted by 0.25 Å from the center of interaction, and with the surrounding first shell argon atoms relaxed into an approximately ellipsoidal cage. The semi-axes of the ellipsoid deviate from the radius of the undistorted first shell, 3.756 Å, by only 0.06 Å. The second shell of the argon atoms is also distorted into an ellipsoidal cage but the deviations from the sphere in this case are an order of magnitude smaller than for the first shell. Distortions of more distant crystal sites are negligible. Substitution of an Ar atom by a CO molecule with subsequent lattice relaxations into the minimum potential energy configuration results in a decrease in potential energy of 0.16 kJ mol^?1. The three-body contributions to the CO·Ar potential have only a negligible influence on the geometry of the COAr system (including the lattice relaxations) but a strong influence on the vibrational frequency shift.