Raman spectroscopic study on the solvation of N,N-dimethyl-p-nitroaniline in room-temperature ionic liquids

Electronic absorption spectra and Raman spectra of N,N-dimethyl-p-nitroaniline (DMPNA) have been measured in various fluids from the gaseous-like conditions in supercritical fluids (SCFs) to highly polar room-temperature ionic liquids (RTILs). We found that the S0-S1 absorption band center of DMPNA in RTILs is mostly determined by the molar concentrations of ions. On the other hand, the bandwidth of the absorption spectrum does not follow the expectation from a simple dielectric continuum model. Especially in SCFs, the bandwidth of the absorption spectrum decreases with increasing solvent density, suggesting that the intramolecular reorganization energy is a decreasing function of the solvent density. The Raman shift of the NO2 stretching mode has been proven to be a good indicator of the solvent polarity; i.e., the vibrational frequency of the NO2 stretching mode changes from 1340 cm-1 in mostly nonpolar solvent such as ethane to 1300 cm-1 in water. The linear relationship between the absorption band center and the vibrational frequency of the NO2 mode, which was observed for conventional liquids in a previous paper (Fujisawa, T.; Terazima, M.; Kimura, Y. J. Chem. Phys. 2006, 124, 184503), holds almost well for all fluids including SCFs and RTILs. On the other hand, the vibrational bandwidth does not show a simple relationship with the absorption band center. The vibrational bandwidths in RTILs are generally larger in comparison with those in conventional liquids with similar polarity scales. Among the RTILs we investigated, the vibrational bandwidth loosely correlates with the molecular size of the anion. A similar dependence on the anion size is also observed for the bandwidth of the absorption spectrum. We have also investigated the excitation wavelength dependence of the Raman shift of the NO2 stretching mode in RTILs. The extent of the dependence on the excitation wavelength in all fluids is well correlated with the vibrational bandwidth.     

Solvent dependence of absorption intensities and wavenumbers of the fundamental and first overtone of NH stretching vibration of pyrrole studied by near-infrared/infrared spectroscopy and DFT calculations

Near-infrared (NIR) and IR spectra were measured for pyrrole in CCl(4), CHCl(3), and CH(2)Cl(2) to study solvent dependence of absorption intensities and wavenumbers of the fundamental and first overtone of NH stretching vibration. It was found that the wavenumbers of the NH fundamental and its first overtone decrease in the order of CCl(4), CHCl(3), and CH(2)Cl(2), which is the increasing order for of the dielectric constant of the solvents. Their absorption intensities increase in the same order, and the intensity increase is more significant for the fundamental than the overtone. These results for the solvent dependence of the wavenumbers and absorption intensities of NH stretching bands of pyrrole are quite different from those due to the formation of hydrogen bonds. Quantum chemical calculations of the wavenumbers and absorption intensities of NH stretching bands by using the 1D Schr?dinger equation based on the self-consistent reaction field (SCRF)/isodensity surface polarized continuum model (IPCM) suggest that the decreases in the wavenumbers of both the fundamental and the overtone of the NH stretching mode with the increase in the dielectric constant of the solvents arise from the anharmonicity of vibrational potential and their intensity increases come from the gradual increase in the slope of the dipole moment function.     

Near IR overtone spectral investigations of liquid phase imidazole and the blue-shifting hydrogen bonds

The near infrared vibrational overtone absorption spectrum of imidazole dissolved in carbon tetrachloride is examined in the region Delta v = 2, 3 and 4. Instead of the standard hydrogen bonding, unusual blue-shifted hydrogen bonds are observed in NH positions. This observation is in agreement with the recent conclusions drawn from theoretical studies of the coupling character between the Imidazole-Imidazolium complex, which is strongly hydrogen bonded. The NH local mode mechanical frequency values and anharmonicity values obtained from fitting the overtones are analysed. The anharmonicity of blue-shifted hydrogen bonded NH stretching bands is greater compared to those of the free NH stretching bands. It is contrary to the decrease in anharmonicity due to the standard red shifting of NH bonds.     

Hydrogen bonding effects on the wavenumbers and absorption intensities of the OH fundamental and the first, second, and third overtones of phenol and 2,6-dihalogenated phenols studied by visible/near-infrared/infrared spectroscopy

Visible, near-infrared (NIR) and IR spectra in the 15600-2500 cm(-1) region were measured for phenol and 2,6-difluorophenol, 2,6-dichlorophenol, and 2,6-dibromophenol in n-hexane, CCl(4), CHCl(3) and CH(2)Cl(2) to study hydrogen bonding effects and solvent dependences of wavenumbers and absorption intensities of the fundamental and the first, second, and third overtones of OH stretching vibrations. A band shift of the OH stretching vibrations from a gas state to a solution state (solvent shift) was plotted versus vibrational quantum number (v = 0, 1, 2 and 3), and it was found that there is a linear relation between the solvent shift and the vibrational quantum number. The slope of solvent shift decreases in the order of phenol, 2,6-difluorophenol and 2,6-dichlorophenol. For all of the solute molecules, the slope becomes larger with the increase in the dielectric constant of the solvents. The relative intensities of the OH stretching vibrations of phenol in CCl(4), CHCl(3), and CH(2)Cl(2) against the intensity of the corresponding OH vibration in n-hexane increase in the fundamental and the second overtone but decrease in the first and third overtones; the relative intensities show so-called "parity". The parity is more prominent for phenol that has an intermolecular hydrogen bonding than for 2,6-dihalogenated phenols that have an intramolecular hydrogen bond. These observations suggest that the intermolecular hydrogen bond between the OH group and the Cl atom plays a key role for the parity and that the intermolecular interaction between the solutes and the solvents (solvent effects) does not have a significant role in the parity.     

Raman spectroscopic study of the solvation of decafluorobenzophenone ketyl radical and related compounds in 2-propanol at ambient to supercritical temperatures

Time-resolved Raman spectroscopy has been applied to the hydrogen-abstraction reaction of decafluorobenzophenone (DFBP) from 2-propanol in temperatures ranging from room to supercritical temperature (520 K) at 31 MPa. The Raman bands of the intermediate ketyl radical (DFBPK) were identified. The Raman bands assigned to the C=C stretching mode (1639 cm-1) and the C-O stretching modes (1274 cm-1) shift to lower frequencies with increasing temperature. The corresponding Raman bands of stable molecules (reference molecules), benzhydrol, decafluorobenzhydrol, and benzophenone (BP), which all have similar molecular structures to those of DFBP or DFBPK, were also investigated at the same range of temperatures. Assignments of the Raman bands were performed with the help of density functional theoretical calculations and the isotopic exchange method. By comparing the Raman peak shifts of the radical with those of the reference molecules, the shift of the C=C stretching mode with increasing temperature (or decrease in the solvent density) is considered to be primarily due to the decrease in the repulsive interaction between the solute and the solvent. On the other hand, the shift of the C-O stretching mode of the radical reflects the decrease in the solvent Lewis acidity or its hydrogen-bonding donating ability, which is clearly illustrated by the shifts of the C=O stretching mode of BP and the C-O stretching mode of 2-propanol. The frequency of the C-O stretching mode of DFBPK was relatively sensitive to the surrounding environment. It was observed that the bandwidth of the radical was generally large, and this observation supports the previous report by Terazima and Hamaguchi (Terazima, M.; Hamaguchi, H. J. Chem. Phys. 1995, 99, 7891). Additionally, the sensitivity and the deformability of the radical structure due to the change of the solvent temperature and density were revealed in our studies.     

Excitation wavelength dependence of the Raman-Stokes shift of N,N-dimethyl-p-nitroaniline

Raman spectra of N,N-dimethly-p-nitroaniline have been measured in various solvents. The Raman-Stokes shift of the band assigned to the NO2 stretching mode excited at 488 nm was found to be linearly dependent on the pi-pi* absorption band center. Furthermore, it is found that the Raman-Stokes shift of the NO2 stretching mode is dependent upon the excitation wavelength. The extent of the shift when excited at 355 versus 488 nm is almost linearly dependent on the vibrational bandwidth of the NO2 mode. The phenomenon is interpreted as the result of the solvation state selective excitation of the vibrational mode as in the case of phenol blue [Yamaguchi et al., J. Chem. Phys. 109, 9075 (1998); 109, 9084 (1998)].     

Attractive and repulsive interactions among methanol molecules in supercritical state investigated by Raman spectroscopy and perturbed hard-sphere theory

The short-range structure of supercritical methanol (CH(3)OH) is investigated by measuring the spontaneous Raman spectra of the C-O stretching mode. The spectra are obtained at a reduced temperature, T(r)=T/T(c)=1.02 (522.9 K), which permits the neat fluid to be studied isothermally as a function of density. As the density increases, the spectral peaks shift toward the lower energy side and the spectra broaden. In the supercritical region, the amount of shifting shows nonlinear density dependence and the width becomes anomalously large. We use the perturbed hard-sphere model to analyze these density dependencies along the vibrational coordinate. The amount of shifting is decomposed into attractive and repulsive components, and the changes in attractive and repulsive energies are evaluated as functions of density and packing fraction, both of which are continuously varied by a factor of 120. Here we show that the shift amount consists principally of the attractive component at all densities, since the attractive energy is about eight times the repulsive energy. The density dependence of the widths is analyzed by calculating homogeneous and inhomogeneous widths as a function of density. The results show that, although vibrational dephasing and density inhomogeneity contribute similarly to the width at low and middle densities, at high density the main contributor turns out to be the vibrational dephasing. We estimate the local density enhancements of supercritical CH(3)OH as function of bulk density by two methods. The results of these analyses show common features, and both the estimated local density enhancements of CH(3)OH are considerably larger than the local density enhancements of simple fluids, i.e., those having nonhydrogen bonding. It is revealed that the local density of supercritical CH(3)OH is 40%-60% greater than the local densities of the simple fluids. We also estimate the local density fluctuation using the obtained values of attractive shift, inhomogeneous width, and local density. The density fluctuation in the vicinity of a vibrating molecule is compared to the fluctuation of bulk density, which is obtained from the thermodynamic calculation.     

CH_3SH和CH_3SLi分子间的锂键和氢键共存型相互作用

在CH3SLi+CH3SH势能面上求得锂键和氢键共存型复合物的两种稳定构型.频率分析表明,与单体相比复合物中S(5)—Li(6)键伸缩振动频率发生红移,而C(8)—H(10)键伸缩振动频率发生蓝移.经B3LYP/6-311++G**,MP2/6-311++G**及MP2/AUG-CC-PVDZ水平计算的含基组重叠误差(BSSE)校正的复合物Ⅰ中相互作用能分别为-58.99,-57.87和-62.89kJ·mol-1.采用自然键轨道(NBO)理论,分析了复合物中单体轨道间的电荷转移,电子密度重排及其与相关键键长变化的本质等.采用分子中的原子(AIM)理论分析了复合物中氢键和锂键的电子密度拓扑性质.在极化连续模型(PCM)下,考察了溶剂化效应.结果表明,所考察的水、二甲亚砜、乙醇和乙醚等四种溶剂均使单体间的相互作用能增大,且溶剂对复合物中的锂键结构及其振动频率具有显著的影响,而对复合物中的氢键的振动频率影响不大.     

Frequencies and absorption intensities of the fundamental and the first overtone of NH stretching vibrations of pyrroleacetylene and pyrroleethylene complexes studied by density-functional-theory calculation

The structures of pyrroleacetylene and pyrroleethylene complexes which form an NH–π hydrogen bonding were calculated by density-functional-theory calculation. The wavenumbers and absorption intensities of the fundamental and the first overtone of NH stretching of the complexes were investigated to compare effects of the hydrogen bonding on the fundamental and the first overtone of the NH stretching mode. One-dimensional Schrödinger equation in consideration of molecular vibrational anharmonicity was used for the estimation of the wavenumbers and absorption intensities of NH stretching modes of the complexes; it was found that the NH–π hydrogen-bond formation induced lower wavenumber shift for both the fundamentals and first-overtones of NH stretching mode and it increases absorption intensities of the fundamentals and decreases those of first overtones of NH stretching mode.     

Vibrational overtone spectra of chloroanilines--evidence for intramolecular hydrogen bonding in o-chloroaniline

The near infrared vibrational overtone absorption spectra of liquid phase aniline and chloroanilines are reported. The analysis of the observed CH and NH local mode mechanical frequency values shows that intramolecular hydrogen bonding occurs between NH2 group and chlorine atom in o-chloroaniline. This observation supports the conclusion drawn from microwave spectroscopic studies reported earlier.     

CH3SH和CH3SLi分子间的锂键和氢键共存型相互作用

在CH₃SLi＋CH₃SH势能面上求得锂键和氢键共存型复合物的两种稳定构型. 频率分析表明, 与单体相比复合物中S(5)—Li(6)键伸缩振动频率发生红移, 而C(8)—H(10)键伸缩振动频率发生蓝移. 经B3LYP/6-311＋＋G**, MP2/6-311＋＋G**及MP2/AUG-CC-PVDZ水平计算的含基组重叠误差(BSSE)校正的复合物?中相互作用能分别为－58.99, －57.87和－62.89 kJ•mol^－1. 采用自然键轨道(NBO)理论, 分析了复合物中单体轨道间的电荷转移, 电子密度重排及其与相关键键长变化的本质等. 采用分子中的原子(AIM)理论分析了复合物中氢键和锂键的电子密度拓扑性质．在极化连续模型(PCM)下, 考察了溶剂化效应. 结果表明, 所考察的水、二甲亚砜、乙醇和乙醚等四种溶剂均使单体间的相互作用能增大, 且溶剂对复合物中的锂键结构及其振动频率具有显著的影响, 而对复合物中的氢键的振动频率影响不大.     

An Infrared Investigation of Hydrogen Bonding of Methanol,Ethanol, 1-Propanol,and 1-Butanol in Solution

The aggregation phenomena of simple alcohols caused by hydrogen bonding in several organic solvents are studied by I. R. spectroscopy. The association constants of cyclic dimer of alcohols in CCI₄ are evaluated, and the importance of solvent bascity effecting on the OH band frequency shift has been discussed.     

共聚焦拉曼光谱研究过饱和硝酸铵液滴中NH4+, NO3- 和H2O之间的相互作用

将硝酸铵液滴沉积在石英基底上,通过降低该液滴周围环境的相对湿度,测定了该液滴由低浓度直至过饱和状态下高信噪比的拉曼光谱.其中,相对湿度的变化可以精确控制液滴浓度的改变.在相对湿度(RH)由72.1%降低至37.9%的过程中,硝酸铵液滴v1-NO-3峰位保持在1048cm-1,半峰宽为10cm-1.该现象表明NO-3周围的水分子被NH4+取代后不会对v1-NO-3造成影响,说明水分子和NH4+所形成的氢键具有相同的强度.对2500-4000cm-1范围内的拉曼光谱进行成分分析,2890、3090、3140、3220、3402及3507cm-1分别被指认为NH+4伞状弯曲振动的泛频、NH+4伞状弯曲振动与摇摆振动的组合谱带、NH+4的对称伸缩振动、NH+4的反对称伸缩振动、水峰中强氢键成分和弱氢键成分.从拟合结果得出:强氢键在氢键结构中所占百分含量随液滴相对湿度的降低而减少,弱氢键所占百分含量随液滴相对湿度的降低而增加.该变化趋势是NO-3和NH+4之间复杂相互作用的结果.     

Hydrogen bonding in supercritical tert-butanol assessed by vibrational spectroscopies and molecular-dynamics simulations

We have investigated the state of aggregation in supercritical tert-butanol (T = 523 K,0.05 < rho < 0.4 g cm(-3)) by means of vibrational spectroscopies (infrared and Raman) and molecular-dynamics (MD) simulations. A quantitative band shape analysis of the spectra associated with the OH stretching mode of tert-butanol has been done using activities computed by ab initio calculations on small clusters. This allows us to determine the degree of hydrogen bonding and populations of oligomers. These latter quantities have been derived from MD simulations and very consistent results are found with experiments. These results show that hydrogen bond still exist in supercritical tert-butanol and that the fluid mainly consists of oligomers smaller than tetramers.     

1H chemical shifts in NMR: Part 23, the effect of dimethyl sulphoxide versus chloroform solvent on 1H chemical shifts

The 1H chemical shifts of 124 compounds containing a variety of functional groups have been recorded in CDCl3 and DMSO-d6 (henceforth DMSO) solvents. The 1H solvent shift Delta delta = delta(DMSO) - delta(CDCl3) varies from -0.3 to +4.6 ppm. This solvent shift can be accurately predicted (rms error 0.05 ppm) using the charge model of alpha, beta, gamma and long-range contributions. The labile protons of alcohols, acids, amines and amides give both, the largest solvent shifts and the largest errors. The contributions for the various groups are tabulated and it is shown that for H.C.C.X gamma-effects (X = OH, NH, =O, NH.CO) there is a dihedral angle dependence of the gamma-effect. The group contributions are discussed in terms of the possible solvent-solute interactions. For protic hydrogens, hydrogen bonding is the dominant interaction, but for the remaining protons solvent anisotropy and electric field effects appear to be the major factors.     

Effects of hydrogen bond and solvent polarity on the C=O stretching of bis(2-thienyl)ketone in solution

The optimized structural parameters, the absorption and the resonance Raman spectra have been investigated for the bis(2-thienyl)ketone in gas phase, in cyclohexane, methanol, and acetonitrile solvents by means of time dependent density functional theory calculations, the solvent electronic polarization effect on the solvation shift is examined and in well accordance with the calculation. The effect of increasing the polarity of the solvent is well represented by the polarizable continuum model, both for the absorption spectra and resonance Raman intensities. The Raman spectra of the C=O stretching mode, which is sensitive to the intermolecular interaction for bis(2-thienyl)ketone dissolved in solvents, were systematically studied. It was found that the hydrogen bond effect plays an important role in reducing the carbonyl stretching wavenumbers. The results of Raman shifts were interpreted through the dilution effect, solvation effects, and hydrogen bond-forming effects. Furthermore, the excitation profiles of several important Raman bands of bis(2-thienyl)ketone molecule in different solvents have been critically analyzed. The solvent effects on structural and symmetry properties of the molecule in S2 electronic state as well as the short-time photo relaxation dynamics have been discussed.     

Vibrational relaxation of azide ions in liquid-to-supercritical water

The dynamics of vibrational energy relaxation (VER) of the aqueous azide anion was studied over a wide temperature (300 K ≤ T ≤ 663 K) and density (0.6 g cm(-3) ≤ ρ ≤ 1.0 g cm(-3)) range thereby covering the liquid and the supercritical phase of the water solvent. Femtosecond mid-infrared spectroscopy on the ν(3) band associated with the asymmetric stretching vibration of the azide anion was used to monitor the relaxation dynamics in a time-resolved fashion. The variation of the vibrational relaxation rate constant with temperature and density was found to be rather small. Surprisingly, the simple isolated binary collision model is able to fully reproduce the experimentally observed temperature and density dependence of the relaxation rate provided a local density correction around the vibrationally excited solute based on classical molecular dynamics simulations is used. The simulations further suggest that head-on collisions of the solvent with the terminal nitrogen atoms rather than side-on collisions with the central nitrogen atom of the azide govern the vibrational energy relaxation of this system. Finally, the importance of hydrogen bonding for the VER dynamics in this system is briefly discussed.     

Dependence of amide vibrations on hydrogen bonding

The effect of hydrogen bonding on the amide group vibrational spectra has traditionally been rationalized by invoking a resonance model where hydrogen bonding impacts the amide functional group by stabilizing its [(-)O-C=NH (+)] structure over the [O=C-NH] structure. However, Triggs and Valentini's UV-Raman study of solvation and hydrogen bonding effects on epsilon-caprolactum, N, N-dimethylacetamide (DMA), and N-methylacetamide (NMA) ( Triggs, N. E.; Valentini, J. J. J. Phys. Chem. 1992, 96, 6922-6931) casts doubt on the validity of this model by demonstrating that, contrary to the resonance model prediction, carbonyl hydrogen bonding does not impact the AmII' frequency of DMA. In this study, we utilize density functional theory (DFT) calculations to examine the impact of hydrogen bonding on the C=O and N-H functional groups of NMA, which is typically used as a simple model of the peptide bond. Our calculations indicate that, as expected, the hydrogen bonding frequency dependence of the AmI vibration predominantly derives from the C=O group, whereas the hydrogen bonding frequency dependence of the AmII vibration primarily derives from N-H hydrogen bonding. In contrast, the hydrogen bonding dependence of the conformation-sensitive AmIII band derives equally from both C=O and N-H groups and thus, is equally responsive to hydrogen bonding at the C=O or N-H site. Our work shows that a clear understanding of the normal mode composition of the amide vibrations is crucial for an accurate interpretation of the hydrogen bonding dependence of amide vibrational frequencies.     

Hydrogen bonding interactions between ethylene glycol and water: density,excess molar volume,and spectral study

Studies of the density and the excess molar volume of ethylene glycol (EG)-water mixtures were carried out to illustrate the hydrogen bonding interactions of EG with water at different temperatures. The re-sults suggest that a likely complex of 3 ethylene glycol molecules bonding with 4 water molecules in an ethylene glycol-water mixture (EGW) is formed at the maximal excess molar volume,which displays stronger absorption capabilities for SO2 when the concentration of SO2 reaches 400×10?6 (volume ratio) in the gas phase. Meanwhile,FTIR and UV spectra of EGWs were recorded at various EG concentra-tions to display the hydrogen bonding interactions of EG with water. The FTIR spectra show that the stretching vibrational band of hydroxyl in the EGWs shifts to a lower frequency and the bending vibra-tional band of water shifts to a higher frequency with increasing the EG concentration,respectively. Furthermore,the UV spectra show that the electron transferring band of the hydroxyl oxygen in EG shows red shift with increasing the EG concentration. The frequency shifts in FTIR spectra and the shifts of absorption bands in UV absorption spectra of EGWs are interpreted as the strong hydrogen bonding interactions of the hydrogen atoms in water with the hydroxyl oxygen atoms of EG.