Solvent Effects on IR Modes of (R)-3-Methylcyclopentanone Conformers: A Computational Investigation

Density Functional Theory (DFT) calculations of infrared spectra for the optimized geometries of the R-(+)-3-methylcyclopentanone (R3MCP) equatorial-methyl and axial-methyl conformers were performed in eleven common solvents with a wide polarity range, within the framework of polarizable continuum model (PCM). The DFT correlation function type B3LYP, using a powerful basis set (aug-cc-pVDZ), yielded different linear correlations between solvent polarity and R3MCP equatorial and axial conformers for frequencies of IR modes, intensities, and enthalpies. DFT calculations of the dipole moments of R3MCP equatorial and axial conformer components in 3D were also carried out and found to have a linear correlation with the solvent polarity. An increasing trend for the hypsochromic (blue) shift in the equatorial conformer??s IR frequencies is observed, in comparison to bathochromic (red) shift for the axial-methyl conformer IR modes, as a function of the solvent polarity.     

Stereoelectronic interaction and their effects on conformational preference for 2-substituted methylenecyclohexane: an experimental and theoretical investigation

Conformational preferences for 2-substituted methylenecyclohexanes were determined using (3) J H 2 H 3 spin-spin coupling constants, while stereoelectronic interactions were obtained by means of theoretical calculations and NBO analysis. The conformational equilibrium of compounds studied can be represented by their axial and equatorial conformers, the axial conformers being the most stable form in polar and nonpolar solvents. These conformational preferences were attributed to the hyperconjugative interactions between the pi C-C-->sigma* C-Xax. and sigma C-H-->sigma* C-Xax. orbitals, and the repulsive steric interaction observed between sigma C-H-->n Xeq..     

Linear and nonlinear circular dichroism of R-(+)-3-methylcyclopentanone

Linear and nonlinear circular dichroism of R-(+)-3-methylcyclopentanone (R-3MCP) is reported in the gas and liquid phases. Measurements of (2+1) resonance-enhanced multiphoton ionization circular dichroism (REMPICD) for nozzle-jet expanded molecular beams of the equatorial conformer of R-3MCP are presented. Monitoring either mass-selected cations or photoelectrons produced via (2+1) REMPI through the n --> 3s Rydberg transition yielded a REMPICD of +1.5+/-0.5% [REMPICD identical with 2(I(L)-I(R))(I(L)+I(R))], where I(L/R) refers to the ion/electron signal for left/right circularly polarized light. A racemic mixture of 3-methylcyclopentanone showed no significant CD; however, the signal fluctuations were much larger than that observed for the resolved R-(+)-3-methylcyclopentanone as might be expected for the small number of ions produced from slightly unequal numbers of enantiomers in each laser shot. Gas phase, vibrationally resolved, one-photon CD for vapor phase R-(+)-3-methylcyclopentanone (i.e., admixture of five axial and equatorial forms) was measured to be approximately 0 and -0.004 at photon energies corresponding to the one- (nonresonant) and two-(3s resonance) photon energy levels. The one-photon CD (of the room temperature population of conformers) at an energy corresponding to the ionization step was measured previously to be approximately +0.0011 which is of the same sign as the REMPICD. The first step is also near a positive CD region. This suggests that the (2+1) REMPICD is determined primarily by both the initial and continuum steps. The one-photon CDs for the equatorial and axial forms of 3MCP are calculated, using GAUSSIAN03, to be approximately equal but having opposite sign for the transitions of interest. The CD for 3MCP in cyclohexane is found to be strongly temperature dependent as a result of the presence of both the axial and equatorial conformers. The energy difference between the two conformers is determined from a van't Hoff plot of these data to be 3.50+/-0.05 kJ/mole in cyclohexane and is approximately 1 kJ/mole smaller than measurements employing other methods.     

The electronic absorption spectra of pyridine azides, solvent-solute interaction

The electronic absorption spectra of: 2-, 3-, and 4-azidopyridines have been investigated in a wide variety of polar and non-polar solvents. According to Onsager model, the studied spectra indicate that the orientation polarization of solvent dipoles affects the electronic spectrum much stronger than the induction polarization of solvent dipoles. The effect of solvent dipole moment predominates that of solvent refractive index in determining the values of band maxima of an electronic spectrum. The spectra of azidopyridines differ basically from these of pyridine or mono-substituted pyridine. Results at hand indicate that the azide group perturbs the pyridine ring in the case of 3-azidopyridine much more than it does in the case of 2-azidopyridine. This result agrees with the predictions of the resonance theory. Although the equilibrium <==> azide tetrazole is well known, yet the observed spectra prove that such an equilibrium does not exist at the studied conditions. The spectra of the studied azidopyridines are characterized by the existence of overlapping transitions. Gaussian analysis is used to obtain nice, resolved spectra. All the observed bands correspond to pi-->pi* transitions, n-->pi* may be overlapped with the stronger pi-->pi* ones.     

Hyperconjugation and the increasing bulk of OCOCX3 substituents in trans-1,4-disubstituted cyclohexanes destabilize the diequatorial conformer

The trans diesters of 1,4-cyclohexanediol with a number of acetic acid analogues, CX3COOH, of varying steric hindrance and polarity (CX3 = Me, Et, iso-Pr, tert-Bu, CF3, CH2Cl, CHCl2, CCl3, CH2Br, CHBr2, CBr3) were synthesized, and the axial,axial/equatorial,equatorial conformational equilibria were studied by low-temperature 1H NMR spectroscopy in CD2Cl2. The structures and relative energies of the axial,axial and equatorial,equatorial conformers were calculated at both the MP2/6-311G* and the MP2/6-311+G* levels of theory, and it was only by including diffuse functions that a good correlation of deltaG(o)calcd vs deltaG(o)exptl could be obtained. Both the structures and the energy differences of the axial,axial and equatorial,equatorial conformers are discussed with respect to the established models of conformational analysis, viz., steric 1,3-diaxial and hyperconjugative interactions. Interestingly, the hyperconjugative interactions sigma(C-C)/sigma(C-H) --> sigma*(C-O), together with a steric effect which also destabilizes the equatorial,equatorial conformers on increasing bulk of the substituents, proved to dominate the position of the conformational equilibria. In addition, the preference of the axial,axial conformers with respect to their equatorial,equatorial analogues was greater than expected from the conformational energies of the corresponding substituents in the monosubstituted cyclohexyl esters. The reason for this very interesting and unexpected result is also discussed.     

Thermal and solvent effects on the NMR and UV parameters of some bioreductive drugs

15N NMR chemical shifts and n-->pi* electronic transition energy for metronidazole (1) has been calculated and compared with experimental data. A detailed computational study of 1 is presented, with special attention to the performance of various theoretical methods for reproducing spectroscopic parameters in solution. The most sophisticated approach involves density functional based on the Car-Parrinello molecular dynamics simulations of 1 in aqueous solution (BP86 level) and averaging chemical shifts and deltaE(n-->pi*) over snapshots from the trajectory. In the NMR and UV calculations for these snapshots (performed at the B3LYP level), a small number of discrete water molecules are retained, and the remaining bulk solution effects are included via a polarizable continuum model (PCM). A good agreement with experiment is also obtained using static geometry optimization and NMR computation of pristine 1 employing a PCM approach. Further theoretical predictions are also reported for 17O NMR and deltaE(n-->pi*) of three hydroxycinnamic acid derivatives, which suggest that it is essential to incorporate the dynamics and solvent effects for NMR and UV calculations in the condensed phase.     

Influence of environment on the electronic structure of Cob(III)alamins: time-resolved absorption studies of the S(1) state spectrum and dynamics

Transient absorption spectroscopy has been used to elucidate the nature of the S1 intermediate state populated following excitation of cob(III)alamin (Cbl(III)) compounds. This state is sensitive both to axial ligation and to solvent polarity. The excited-state lifetime as a function of temperature and solvent environment is used to separate the dynamic and electrostatic influence of the solvent. Two distinct types of excited states are identified, both assigned to pi3d configurations. The spectra of both types of excited states are characterized by a red absorption band (ca. 600 nm) assigned to Co 3d --> 3d or Co 3d --> corrin pi* transitions and by visible absorption bands similar to the corrin pi-->pi* transitions observed for ground state Cbl(III) compounds. The excited state observed following excitation of nonalkyl Cbl(III) compounds has an excited-state spectrum characteristic of Cbl(III) molecules with a weakened bond to the axial ligand (Type I). A similar excited-state spectrum is observed for adenosylcobalamin (AdoCbl) in water and ethylene glycol. The excited-state spectrum of methyl, ethyl, and n-propylcobalamin is characteristic of a Cbl(III) species with a sigma-donating alkyl anion ligand (Type II). This Type II excited-state spectrum is also observed for AdoCbl bound to glutamate mutase. The results are discussed in the context of theoretical calculations of Cbl(III) species reported in the literature and highlight the need for additional calculations exploring the influence of the alkyl ligand on the electronic structure of cobalamins.     

On the performance of quantum chemical methods to predict solvatochromic effects: the case of acrolein in aqueous solution

The performance of the Hartree-Fock method and the three density functionals B3LYP, PBE0, and CAM-B3LYP is compared to results based on the coupled cluster singles and doubles model in predictions of the solvatochromic effects on the vertical n-->pi* and pi-->pi* electronic excitation energies of acrolein. All electronic structure methods employed the same solvent model, which is based on the combined quantum mechanics/molecular mechanics approach together with a dynamical averaging scheme. In addition to the predicted solvatochromic effects, we have also performed spectroscopic UV measurements of acrolein in vapor phase and aqueous solution. The gas-to-aqueous solution shift of the n-->pi* excitation energy is well reproduced by using all density functional methods considered. However, the B3LYP and PBE0 functionals completely fail to describe the pi-->pi* electronic transition in solution, whereas the recent CAM-B3LYP functional performs well also in this case. The pi-->pi* excitation energy of acrolein in water solution is found to be very dependent on intermolecular induction and nonelectrostatic interactions. The computed excitation energies of acrolein in vacuum and solution compare well to experimental data.     

Testing computational models of hyperpolarizability in a merocyanine dye using spectroscopic and DFT methods

The structural and electronic properties of a highly solvatochromic merocyanine dye, 2-(3-cyano-5,5-dimethyl-4-(3-(1-octadecylpyridin-4(1H)-ylidene)prop-1-enyl)furan-2(5H)-ylidene)malononitrile (pyr3pi), have been investigated using UV-vis, NMR, hyper-Rayleigh scattering, and Raman spectroscopies and further interpreted using computational chemistry. Spectroscopic data indicate that pyr3pi exists in its zwitterionic form even in low polarity solvents with electronic absorption spectra showing a hypsochromic shift with an increase in solvent polarity and NMR experiments indicating an increasingly zwitterionic structure in chloroform as the temperature is lowered. Raman spectra in increasingly polar solvents show small variations of the structure that are consistent with a change toward a structure with more zwitterionic character. However, comparison of the calculated and experimental vibrational energies and intensities and comparison of NMR coupling constants with calculated bond order indicate that calculations underestimate the amount of charge separation seen in low polarity solvents. Although for this system density functional theory (DFT) calculations and the two-state model qualitatively reproduce negative solvatochromism, they fail to reproduce the trends in hyperpolarizability seen experimentally. This is attributed to solvent field DFT calculations underestimating the degree of charge separation in reaction fields representing low polarity solvents.     

The utility of infrared spectroscopy for quantitative conformational analysis at a single temperature

The carbonyl stretching vibration of 2-bromocyclohexanone (1) has been measured in a variety of solvents. It is shown that its component intensities are not only dependent on the populations of the axial and equatorial conformers, but are also dependent on the molar absorptivities (epsilon ) which are specific for each conformer in each solvent. In CCl(4), the axial and equatorial conformers have epsilon values of 417 and 818 l mol(-1) x cm(-1), respectively, while in CH(3)CN solution, the values were 664 and 293 l mol(-1) x cm(-1). These results are supported by results of theoretical calculations of frequencies, which gave an intensity of 223.8 kM mol(-1) x(1782 cm(-1)) for the axial and 174.4 kM mol(-1) x (1802 cm(-1)) for the equatorial conformer, indicating that the axial conformer presents a larger molar absorptivity than the equatorial one in the vapor phase. Moreover, the results presented here clearly demonstrate that although infrared spectroscopy at a single temperature can be an important auxiliary technique for conformational analysis, it must not be used to quantify conformational preferences of a molecule if the absorption molar coefficients for each conformer are not known or not amenable to experimental determination.     

Solvatochromic behavior of donor-acceptor-polyenes: dimethylamino-cyano-diphenylbutadiene

4-(Dimethylamino)-4'-cyano-1,4-diphenylbutadiene (DCB) and 4-(dimethylamino)-2,6-dimethyl-4'-cyano-1,4-diphenylbutadiene (DMDCB) have been characterized spectroscopically. Quantum chemical calculations were performed for comparison. Solvatochromic shifts of the fluorescence were strong and showed a linear dependence on the solvent polarity parameters, whereas shifts in the absorption spectra are very weak only correlate better with the polarizability of the solvents. Excited state dipole moments derived from fluorescence using the Onsager model are very large and similar for both compounds. It is concluded that a strongly allowed and highly dipolar pi, pi* state is the lowest excited state in polar solvents. The strong difference in absorption and fluorescence solvatochromic slopes suggests that the simple Onsager model with a point dipole approximation is not sufficient here.     

Conformation equilibrium of 3-(hydroxymethyl)piperidine in solvents with different polarity

Quantum-chemical calculations of the 3-(hydroxymethyl)piperidine molecule conformers were performed at the B3LYP/6-31+G** level of theory, and four most stable conformations with different relative orientation of CH₂OH and N–H groups were determined. The optimized structures, vibration frequencies, and band intensities in the spectra of the conformers were obtained. The conformational equilibria of the most stable rotational isomers in solvents of different polarity was studied within the polarizable continuum model. According to the results of calculations, the conformational equilibrium in solution is substantially changed on varying the solvent polarity. This conclusion was confirmed by comparison with IR absorption spectra of 3-(hydroxymethyl)piperidine solutions in carbon tetrachloride in the region of ОН-stretchings.

     

Ab initio n-electron valence state perturbation theory study of the adiabatic transitions in carbonyl molecules: formaldehyde, acetaldehyde, and acetone

The application of the recently developed second-order n-electron valence state perturbation theory (NEVPT2) to small carbonyl molecules (formaldehyde, acetaldehyde, and acetone) is presented. The adiabatic transition energies are computed for the singlet and triplet n-->pi(*), pi-->pi(*), and sigma-->pi(*) states performing a full geometry optimization of the relevant states at the single state CASSCF level and taking into account the zero point energy correction in the harmonic approximation. The agreement with the known experimental values and with previously published high level calculations confirms that NEVPT2 is an efficient tool to be used for the interpretation of molecular electronic spectra. Moreover, different insight into the nature of the excited states has been obtained. Some of the transitions presented here have never been theoretically computed previously [(3)(pi-->pi(*)) and (3)(sigma-->pi(*)) adiabatic transitions in acetaldehyde and acetone] or have been studied only using moderate level (single reference based) ab initio methods (all adiabatic transitions in acetaldehyde). In the present work a consistent disagreement between NEVPT2 and experiment has been found for the (3)(pi-->pi(*)) adiabatic transition in all molecules: this result is attributed to the low intensity of the transition to the first vibrational levels of the excited state. The n-->pi(*) singlet and triplet vertical transition energies are also reported for all the molecules.     

Photochemical electron transfer reactions of tirapazamine

The absorption and fluorescence spectra of 3-aminobenzo-1,2,4-triazine di-N-oxide (tirapazamine) have been recorded and exhibit a dependence on solvent that correlates with the Dimroth ET30 parameter. Time-dependent density functional theory calculations reveal that the transition of tirapazamine in the visible region is pi-->pi* in nature. The fluorescence lifetime is 98+/-2 ps in water. The fluorescence quantum yield is approximately 0.002 in water. The fluorescence of tirapazamine is efficiently quenched by electron donors via an electron-transfer process. Linear Stern-Volmer fluorescence quenching plots are observed with sodium azide, potassium thiocyanate, guanosine monophosphate and tryptophan (Trp) methyl ester hydrochloride. Guanosine monophosphate, tyrosine (Tyr) methyl ester hydrochloride and Trp methyl ester hydrochloride appear to quench the fluorescence at a rate greater than diffusion control implying that these substrates complex with tirapazamine in its ground state. This complexation was detected by absorption spectroscopy.     

Optical absorption of 1H-pyrazolo[3,4-b]quinoline and its derivatives

The results of experimental studies and quantum chemical simulations of the absorption spectra of 1H-pyrazolo[3,4-b]quinoline and its derivatives are presented. The quantum chemical calculations (semi-empirical AM1 and PM3 methods) show similarity in the absorption spectra of 1H-pyrazolo[3,4-b]quinoline and 1,3-dimethyl-1H-pyrazolo[3,4-b]quinoline which are characterized by five strong absorption bands in the spectral range 200-500 nm. A substitution of the methyl groups by at least one phenyl group causes the drastic changes of the absorption spectra mainly within the spectral range 240-370 nm. We attribute these differences to additional molecular double bonding segments C=C of the substituted phenyl groups, i.e. to pi --> pi* transitions. A comparison of measured and the calculated absorption spectra manifests quite satisfactory agreement for all compounds in the part regarding the spectral position of the first oscillator (absorption threshold). At the same time, the measured spectra demonstrate the considerable broadening practically of all absorption bands and even complete damping some of them in the case of phenyl derivatives. The experiments performed with highly and weakly polar organic solvents shows that the solvent effect on the absorption spectra is small. For this reason the discrepancies between the calculated and the measured spectra are attributed to electron-vibronic coupling as well as to rotational dynamics of phenyl rings.     

Electronic absorption spectra of benzoquinone and its hydroxy substituents and effect of solvents on their spectra

The electronic absorption spectra of 1,4-benzoquinone (BQ) and its 2,5-dihydroxy and tetrahydroxy derivatives have been studied in detail. The interpretation of the electronic bands is made on the basis of PPP and CNDO calculations. It is found that the pi --> pi* transitions are well predicted by the PPP method. The predictions of the CNDO method are however superior both in their accuracy as well as ability to predict the n --> pi* transitions. The effect of solvents on the electronic absorption bands have also been investigated in detail. Linear correlations are found between the solvent's dielectric constant and wavelength of the absorption bands. The solvent shifts are explained on the basis of the polarities of the solute and solvent molecules as well as due to hydrogen bonding.     

Solvent effect on absorption and fluorescence spectra of coumarin laser dyes: evaluation of ground and excited state dipole moments

The effect of solvents on absorption and fluorescence spectra and dipole moments of coumarin 307 (C307) and coumarin 522B (C522B) have been studied extensively in various solvents, viz., general solvents, alcohols and binary mixtures (acetonitrile-benzene) at 298K. The bathchromic shift observed in absorption and fluorescence spectra of C307 and C522B with increasing solvent polarity indicates that transition involved are pi-->pi*. Solvatochromic correlations were used to obtain the ground and excited state dipole moments. The excited state dipole moments are observed to be greater than their ground state counterparts in all the solvents studied. Further, the experimentally obtained Deltamu were compared with those using normalized polarity terms E(T)(N) from Reichardt equation.