Rotational Isomers Research Articles

Improved descriptions of collective and non-collective rotations in the superheavy nucleus 256Rf

The superheavy nucleus 256Rf, where rotational band and multi-quasiparticle isomer have been observed recently, has been investigated using total Routhian surface calculations and configuration-constrained calculations of potential energy surface, with the inclusion of β 6 deformation. The experimental moment of inertial is well reproduced, indicating that the alignment is delayed due to the β 6 deformation. A K π = 5− or 8− state could form a two-quasiparticle isomer that is calculated to have higher fission barrier than the ground state.

Phosphorescence emission and excited states of 3- and 4-hydroxybenzaldehyde vapors

Emission spectra of 3- and 4-hydroxybenzaldehyde vapors have been measured along with the excitation and absorption spectra. It is inferred from the temperature dependence of the phosphorescence spectrum that the phosphorescence of 4-hydroxybenzaldehyde vapor originates from the T2(n,π*) state. In the case of 3-hydroxybenzaldehyde vapor, the emission is shown to consist of that of 3-hydroxybenzaldehyde and benzaldehyde vapors, with the latter molecule being generated as the result of photochemical decomposition of 3-hydroxybenzaldehyde. The interpretation of the emission spectrum of 3-hydroxybenzaldehyde is different from that reported previously. The rotational isomer responsible for appearance of the phosphorescence of 3-hydroxybenzaldehyde vapor is suggested based on DFT calculation and emission spectral data.

Vacuum Ultraviolet and Soft X‐ray Photoelectron Spectroscopy of Liquid Beams Using a Hemispherical Photoelectron Spectrometer with a Multistage Differential Pumping System

AbstractWe present photoelectron spectroscopy of liquid beams using a high‐resolution hemispherical photoelectron spectrometer with a multistage differential pumping system developed in our laboratory. Intense soft X‐ray synchrotron radiation (500‐1200 eV) at SPring‐8 and a relatively large detection solid angle (1.5 × 10−2 π sr) of the spectrometer enable highly efficient measurements of X‐ray photoelectron spectra of liquids. Vacuum ultraviolet photoelectron spectroscopy using the same spectrometer provides sufficiently high spectral resolution to resolve rotational isomers of gas‐phase molecules.

Dynamic 1H NMR studies around the heteroaryl–carbon (N CH−N) and nitrogen–carbon (N C=CH) single bonds in a particular enaminoester involving a phenanthridine along with theoretical calculations

For the first time, a series of separate dynamic effects are reported at different temperatures within a particular enaminoester 4 involving a phenanthridine namely dimethyl-2-(6-(2-methylindol-1-yl)phenanthridine-5(6H)-yl) maleate. These effects are attributed to restricted rotation around the two single bonds such as heteroaryl–carbon (N CH–N) and nitrogen–carbon (N C=CH). Rotational energy barriers (ΔG ‡) for these interconversion processes are 44.88 and 44.63 ± 1 kJ mol−1, respectively, in rotational isomers. In addition, theoretical studies on the basis of rotational around the same bonds were investigated using an ab initio method at HF/6-31G(d,p) and B3LYP/6-31G(d,p) levels of theory. Theoretical activation parameters including ΔH ‡, ΔG ‡, ΔS ‡, and kinetic parameters involving E a were calculated in the presence of a solvent medium (CDCl3) at experimental temperature. The results indicated that the HF/6-31G(d,p) level provides more accurate data in comparison with the B3LYP method with the same basis set [6-31G(d,p)] for compound 4 and they are in relative agreement with dynamic 1H NMR data.

Modulation of charge transfer rate in (N,N′-dimethylamino)benzonitrile liquid solutions

We studied the properties of the emission, absorption and excitation of dual fluorescence of (N,N′−dimethylamino)benzonitrile in a polar aprotic solvent acetonitrile under selective irradiation of solutions by light with different energies of quanta to elucidate mechanisms of dual fluorescence arising in this solvent at different temperatures in the range 274–313 K. In all cases, dual fluorescence of the solute in acetonitrile was observed, which is caused by emission from locally excited Franck-Condon and charge-transfer states. A change in the energy of excitation quanta has a weak effect on the position of the fluorescence bands; however, the intensity ratio between the bands noticeably changes in favour of the intensity of the long-wavelength band at excitation in the range of the long-wavelength absorption band. An interesting and unusual fact is that solution heating is accompanied by essential growth of quantum yield of dual fluorescence at all wavelengths of the excitation. To explain the observed effects, the same dependences were measured and analysed for DMABN in neutral solvent n-hexane in the same conditions. We involve also the data of quantum-mechanical calculations, which show that there is a considerable probability of occurrence in solutions of DMABN rotational isomers with differing orientation of the dimethylamino group with respect to the benzonitrile. In the excited state, these have different charge-transfer rates, resulting in a modulation in the intensity ratio of the observed fluorescence bands with change excitation energy quanta on the red wing of the absorption band, doi: 10.1134/S0030400X12050219.

Synthesis and mesomorphic investigation of calamitic liquid crystalline system ethyl-[4-(4′-decyloxy)benzoyloxy]-benzoate (4-EDBB): A temperature dependent micro-Raman study and DFT calculations

A new liquid crystalline material containing diester linking group ethyl-[4-(4′-decyloxy)benzoyloxy]-benzoate (4-EDBB) was synthesized. The phase transition temperatures were noted by differential scanning calorimetry (DSC), the texture pattern were observed by polarizing optical microscopy (POM) and temperature dependent Raman study was employed to observe the transitions as well as to understand the molecular rearrangement during phase transition. The transitions were observed with all the three techniques but the Raman signature of crystal→smectic A transition is many fold and more precise and accurate. The correlation between intermolecular interaction and phase behaviour has been discussed using temperature dependence Raman data of CH in-plane bending and CO stretching vibrations. With the help of DFT method the possible dimers of 4-EDBB were optimized and the rotational isomers were also investigated. There exists weak hydrogen bonds at room temperature, which breaks as the temperature is increased causing the CH in-plane bending to shift lower and CO stretching vibrations to shift higher. The discussion of the temperature dependent Raman data reveals that at crystal→smectic A transition as a result of intra-molecular rotation the molecules transform from trans- to cis- conformer.

Structural Modification on Copper(I)-pyridylpyrimidine Complexes for Modulation of Rotational Dynamics, Redox Properties, and Phototriggered Isomerization

The redox properties of copper pyridylpyrimidine complexes, which undergo linkage isomerism based on pyrimidine ring rotation, were compared under different coordination environments. A newly synthesized compound, [Cu(Mepypm)(L(Mes))]BF4 (1·BF4, Mepypm = 4-methyl-2-(2'-pyridyl)pyrimidine, L(Mes) = 2,9-dimesityl-1,10-phenanthroline) was compared with previously reported complexes of [Cu(MepmMepy)(L(Mes))]BF4 (2·BF4, MepmMepy = 4-methyl-2-(6'-methyl-2'-pyridyl)pyrimidine), Cu(Mepypm)(DPEphos)]BF4 (3·BF4, DPEphos = bis[2-(diphenylphosphino)phenyl]ether), [Cu(Mepypm)(L(Anth))]BF4 (4·BF4, L(Anth) = 2,9-bis(9-anthryl)-1,10-phenanthroline), and [Cu(Mepypm)(L(Macro))]BF4 (5·BF4). Isomer ratios, isomerization dynamics, redox properties, and photoelectron conversion functions varied with the coordination structure. Methyl substituents on the 6-position of the pyridine moiety increased steric repulsion and contributed to quicker rotation, enhanced photoluminescence, and increased photodriven rotational isomerization.

The influence of temperature on red-edge excitation effects in liquid solutions of N,N′-dimethylaminobenzonitrile

A systematic study of new photophysical and photochemical processes in solutions is continued by the example of a recently found phenomenon of redistribution of the intensities of two fluorescence bands of N,N′-Dimethylaminobenzonitrile (DMABN) in polar solutions at room temperatures under selective irradiation by light with different photon energies in the region of the long-wavelength absorption band. The effects observed are explained using data of quantum-mechanical calculations, which reveal that solutions of these systems are very likely to contain rotational isomers with different orientations of the dimethylamino group with respect to the plane of the benzonitrile residue. The excited-state charge transfer reactions in these rotamers occur in different ways and, hence, with different rates, because of which the intensity ratio of recorded fluorescence bands is different for different wavelengths of selective excitation. In this study, the influence of the temperature on the red-edge excitation effect observed in the fluorescence of DMABN solutions in acetonitrile is studied in the temperature range of 274–313 K using the previously used selective excitation method. It is found that these effects manifest themselves at any temperature within this range, but are especially strong at 313 K. The parameters of the dual fluorescence that are most sensitive for recording of the considered effects are determined, and the obtained temperature dependences are interpreted.

Rotational isomers, spectroscopic (FT-IR, FT-Raman) studies and quantum chemical calculations on 2,4,6-tris(dimethylaminomethyl) phenol

In this work, the spectroscopic characterization of 2,4,6-tris(dimethylaminomethyl) phenol; a novel promoter factor for DNA has been studied primarily. The FT-IR (4000–400cm−1) and FT-Raman (3500–100cm−1) spectra have been recorded on the solid phase of the title molecule. The spectroscopic signature of the title molecule has been found by comparing experimental FT-IR, FT-Raman spectra with the theoretical IR and Raman spectra of the stable isomer geometry at density functional theory (DFT) method with 6-311++G(d,p) basis set. Further, the vibrational assignments were performed on the basis of potential energy distribution (PED). The natural atomic orbital and natural population analysis performed in this study ensures us to know about the delocalization of charge and electron density of atoms within the molecule. Analysis of natural bond orbitals (NBOs) and HOMO–LUMO energy gap of the compound provides information about its chemical stability and intramolecular charge transfer properties. In addition, the reacting electrophilic and nucleophilic sites of the molecule were predicted with the help of molecular electrostatic potential (MEP) surface analysis. Moreover, the intensity of molecular vibrations at different temperatures were examined by applying thermo-chemical analysis. To investigate the solvent effect, the polarizable continuum model was used and the allowed transitions between various HOMO and LUMO levels were found.

Rotational Isomers of N-Methyl-N-arylacetamides and Their Derived Enolates: Implications for Asymmetric Hartwig Oxindole Cyclizations

The rotational preferences of N-(2-bromo-4,6-dimethylphenyl)-N-methyl 2-phenylpropanamide were studied as a model of precursors for Hartwig asymmetric oxindole cyclizations. The atropisomers of this compound were separated by flash chromatography, and then the enantiomers were resolved and the interconversions of the stereocenter and the N-Ar axis were studied. Under thermal conditions, the axis is very stable. Under the basic conditions of the Hartwig cyclization, both the stereocenter and the chiral axis equilibrate via enolate formation. The N-Ar rotation barrier of a 2-phenylacetamide analogue was reduced from 31 kcal mol(-1) in the precursor to 17 kcal mol(-1) in the enolate. Reasons for this dramatic barrier reduction and implications of both N-Ar and amide C-N rotations for Hartwig cyclizations are discussed.

Spectroscopic Characterization and Constitutional and Rotational Isomerism of ClC(O)SCN and ClC(O)NCS

Chlorocarbonylthio- and isothiocyanate (ClC(O)SCN and ClC(O)NCS) have been isolated and characterized by IR (Ar matrix, gas), Raman (liquid), (13)C NMR and UV-visible spectroscopies. Vibrational and quantum chemical studies suggest the presence of the syn and anti conformers (SCN group with respect to the C═O bond) in the gas phase for both constitutional isomers. syn-ClC(O)SCN is preferred by ΔH° (anti/syn) = 1.3(0.3) kcal mol(-1). The solid-state structure of ClC(O)SCN has been determined by single crystal X-ray diffraction analysis at low temperature. The crystalline solid consists exclusively of molecules in the syn conformation. On the other hand, the anti form is more stable for the ClC(O)NCS isomer. The structure of ClC(O)NCS and its conformational composition were determined by gas electron diffraction. An unusual low syn → anti interconversion energy barrier of 0.98 (0.15) kcal mol(-1) was detected for ClC(O)NCS at cryogenic temperatures. The photochemistry of both constitutional isomers isolated in solid argon at 15 K was studied. Rearrangement of ClC(O)SCN to ClC(O)NCS was observed in the neat liquid and under UV-vis irradiation of ClC(O)SCN isolated in solid argon. Properties have been discussed in terms of the valence electronic structure, including the analysis of the He(I) photoelectron spectrum of ClC(O)SCN.

Ultrasonic Relaxation Measurements in Aqueous Solution and Molecular Orbital Calculation on Imipramine

Ultrasonic absorption coefficients have been measured in aqueous solution of imipramine {3-(10,11-dihydro-5H-dibenzo[b,f]azepin-5-yl)-N,N-dimethylpropan-1-amine} in the frequency range of 0.8-220 MHz at 25 °C. The frequency dependences of the observed absorption was characterized by a Debye-type relaxational equation with two relaxation frequencies, although only one relaxation had been observed in aqueous solutions of the related molecule amitriptyline. Both of the relaxation frequencies in imipramine solutions were found to be independent of the solute concentration and the amplitudes of the relaxational absorptions increase linearly with increasing solute concentration. It was therefore concluded that these two relaxations are associated with unimolecular reactions, such as a structural change due to rotational motions of the bond in the specified group in the imipramine molecule. To analyze quantitatively the source of the relaxations, semiempirical molecular orbital methods have been applied to determine the standard enthalpy of formation of the imipramine molecule at various dihedral angles around the bonds in the alkylamine side chain. According to the results, only one rotational motion of carbon-carbon bond in the side chain was found to be appropriate and the three minima of the standard enthalpy of formation was obtained as a function of the rotational angle. At the three minimum positions, the values of the standard enthalpy of formation are almost the same. With the assumptions (a) that rotational motion is not accompanied by a volume change of the reaction and (b) that the standard free energy change is close to the difference in the values between the standard enthalpies of formation, the equilibrium constants for the rotational isomerization have been calculated to be near unity. Hence, the forward and backward rate constants of the isomerization reactions are nearly the same. If one assumes that there are two kinds of rotational motions in one bond of the molecule, one proceeds with a rate constant on the order of 10(8) s(-1), whereas the other with a rate constant on the order of 10(6) s(-1). The faster and slower processes are also distinguished by the height of the standard enthalpy of formation.

Fluorescence Properties of (E,E,E)-1,6-Di(n-naphthyl)-1,3,5-hexatriene (n= 1, 2): Effects of Internal Rotation

The fluorescence spectroscopic properties of (E,E,E)-1,6-di(n-naphthyl)-1,3,5-hexatrienes (1, n = 1; 2, n = 2) have been investigated in solution and in the solid state. In solution, the absorption maxima (λ(a)) of the lowest-energy band (1, 374 nm; 2, 376 nm in methylcyclohexane) were similar for 1 and 2, whereas the fluorescence maxima (λ(f)) (1, 545 nm; 2, 453 nm) and quantum yields (φ(f)) (1, 0.046; 2, 0.68) were very different regardless of the solvent polarity. The fluorescence spectrum of 1 was independent of the excitation wavelength (λ(ex)), whereas the spectrum of 2 was weakly λ(ex)-dependent. In the solid state, the spectroscopic properties of 1 and 2 were similar (λ(a) = 437-438 nm, λ(f) = 496-505 nm, φ(f) = 0.04-0.07). The origins of emission are both considered to be mainly monomeric. With the help of single-crystal X-ray structure analysis and ab initio quantum chemical calculation, we conclude that the red-shifted and weak emission of 1 in solution originates from a planar excited state having small charge transfer character, reached from a twisted Franck-Condon state by the excited-state geometrical relaxation accompanied by the internal rotation around the naphthalene (Ar)-CH single bond. The similar fluorescence properties of 1 and 2 in the solid state can be attributed to the restriction of the geometrical relaxation. The effects of the Ar-CH rotational isomerism on the fluorescence properties in solution, for 2 in particular, are also discussed.

DFT study of intramolecular interring η6,η6-haptotropic rearrangements in tricarbonylchromium complexes of 2-aminobiphenyl and 4-aminobiphenyl

DFT studies have been carried out to investigate the role of nitrogen participation in the interring haptotropic rearrangements of [(η(6)-C6H5)(C6H4-4-NH2)]Cr(CO)3 and [(η(6)-C6H5)(C6H4-2-NH2)]Cr(CO)3. For the para-substituted case, where intramolecular coordination of nitrogen to chromium is not possible, DFT modeling predicts an activation barrier of 32.5 kcal mol(-1), which is in very close agreement with the experimentally determined value of 32.57 kcal mol(-1). In the case of the ortho-substituted isomers, modeling of a mechanism that does not invoke stabilization via nitrogen coordination yields a predicted energy barrier of 32.7 kcal mol(-1), while a mechanism that does invoke nitrogen participation and accounts for interconversion of rotational isomers gives a predicted value of 30.2 kcal mol(-1). This is consistent with the experimentally determined value of 31.22 kcal mol(-1). These data provide evidence that intramolecular stabilization via nitrogen coordination to chromium is responsible for the ortho substituted isomer undergoing haptotropic rearrangement with a rate nearly five times greater than that observed for the para isomer. For the mechanism that invokes ortho-amino group participation, transition state analysis in the frame of Bader theory shows that each transition state along the proposed mechanistic pathway has a bond critical point between nitrogen and chromium.

Ultrafast ring-opening reactions: a comparison of α-terpinene, α-phellandrene, and 7-dehydrocholesterol with 1,3-cyclohexadiene

The excited and ground state dynamics of a series of 1,3-cyclohexadiene derivatives and their hexatriene photoproducts are studied using ultrafast broadband UV-visible transient absorption spectroscopy. The substitution pattern around the cyclohexadiene backbone alters the excited state potential energy surface in the Franck-Condon region as evidenced by changes in the excited state absorption and fluorescence properties of the systems. Unsubstituted 1,3-cyclohexadiene and alpha-phellandrene exhibit no excited state absorption while a strong excited state absorption in the visible spectral region is observed for both alpha-terpinene and Provitamin D3. Steric factors introduced by the ring substitutions determine the dominant rotational isomer at equilibrium for the hexatriene photoproducts. Coupling to the solvent during the conformational relaxation from the initial helical all-cis hexatriene is unique to each photoproduct, but the relaxation process occurs on a 6-10 ps timescale regardless of the size or substitution pattern on the triene.

Mechanism investigation of the reaction between triphenylphosphine, dialkyl acetylenedicarboxylates and 2-hydroxybenzimidazole: Experimental and theoretical study

In the recent work, NMR study and the stability of the Z- and the E- isomers were undertaken for the two rotational isomers of phosphorus ylides involving 2-hydroxybenzimidazole [namely dimethyl-2-(2-hydroxybenzimidazole-n-yl)-3-(triphenylphosp hanylidene) succinate] by natural population analysis (NPA) and atoms in molecules (AIM) methods. Quantum mechanical calculation was clarified how the ylides exist in solution as a mixture of the two geometrical isomers (Z- and E-) as a minor or major form. In addition, Kinetic studies were performed for the reaction between triphenylphosphine and dialkyl acetylenedicarboxilate in the presence of NH– heterocyclic compound, such as 2-hydroxybenzimidazole. To determine the kinetic parameters of the reaction, it was monitored by UV spectrophotometry. Useful information was obtained from studies of the effect of solvent and concentration of reactants on the rate of reaction. Proposed mechanism was confirmed according to the obtained results and steady state approximation and first step (k2) and third step of reaction were recognized as a rate determining step and fast step, respectively on the basis of experimental data.

Solvated-Ion-Pairing-Sensitive Molecular Bistability Based on Copper(I)-Coordinated Pyrimidine Ring Rotation

We describe herein the effect of solvated ion pairing on the molecular motion of a pyrimidine ring coordinated on a copper center. We synthesized a series of heteroleptic copper(I) complex salts bearing an unsymmetrically substituted pyridylpyrimidine and a bulky diphosphine. Two rotational isomers of the complexes were found to coexist and interconvert in solution via intramolecular ligating atom exchange of the pyrimidine ring, where the notation of the inner (i-) and outer (o-) isomers describes the orientation of the pyrimidine ring relative to the copper center. The stability of the pyrimidine orientation was solvent- and counterion-sensitive in both 2·BF(4) {2(+) = [Cu(Mepypm)(dppp)](+), where Mepypm = 4-methyl-2-(2'-pyridyl)pyrimidine and dppp = 1,3-bis(diphenylphosphino)propane} and previously reported 1·BF(4), which possesses a bulky diphosphine ligand (1(+) = [Cu(Mepypm)(DPEphos)](+), where DPEphos = bis[2-(diphenylphosphino)phenyl] ether). Two rotational isomers of 2(+) were separately obtained as single crystals, and the structure of each isomer was examined in detail. Both the enthalpy and entropy values for the rotation of 2·BF(4) in CDCl(3) (ΔH = 6 kJ mol(-1); ΔS = 25 J K(-1) mol(-1)) were more positive than that tested under other conditions, such as in more polar solvents CD(2)Cl(2), acetone-d(6), and CD(3)CN. The reduced contact of the anion to the cation in a polar solvent seems to contribute to the enthalpy, entropy, and Gibbs free energy for rotational isomerization. This speculation based on solvated ion pairing was further confirmed by considering the rotational behavior of 2(+) with a bulky counterion, such as B(C(6)F(5))(4)(-). The findings are valuable for the design of molecular mechanical units that can be readily tuned via weak electrostatic interactions.

Dynamic1H NMR investigation along with a theoretical study around the C-C and C = C bonds in a particular phosphorus ylide

In the present work, the dynamic 1H NMR effects were investigated at variable temperatures within a particular phosphorus ylide involving a 2-benzoxazolinone around the carbon–carbon single bond and also partial carbon–carbon double bond in two Z- and E-rotational isomers. Activation and kinetic parameters including ΔH≠, ΔG≠, ΔS≠ and Ea were determined in accord with the dynamic 1H NMR data for three rotational processes. In addition, theoretical studies based upon rotation around the same bonds were investigated using ab initio and DFT methods at the HF/6-31G(d,p) and B3LYP/6-31G(d,p) levels of theory. Theoretical activation and kinetic parameters including ΔH≠, ΔG≠, ΔS≠ and Ea were calculated at 298 K and experimental temperatures for five rotational processes. These results (experimental and theoretical), taken together, indicate that the rotational energy barrier around the C = C double bond was considerably high and the observation of the two rotational isomers was impossible (seen as a single isomer) at the high temperatures, in this case rotation around the C = C bond was too fast on the NMR time scale. When the temperature was relatively low, the rate of rotation was sufficiently slow; therefore, observation of the two Z- and E-isomers was possible. In addition, calculations at the HF/6-31G(d,p) level of theory showed very favorable results in agreement with the experimental data on rotation around the C = C bond. While, B3LYP level using the 6-31G(d,p) basis set was provided the reasonable data for the restricted rotations around the C–C and C–N single bond. Copyright © 2012 John Wiley & Sons, Ltd.

Matrix isolation infrared spectroscopic and quantum chemical studies on the rotational isomers of orotic acid (6-carboxyuracil)

The infrared spectrum of orotic acid (6-carboxyuracil) isolated in a low-temperature argon matrix is presented, for the first time. This molecule is a key precursor in the biosynthesis of all pyrimidine nucleotides in living organisms. The comprehensive theoretical studies on the rotational isomerism of orotic acid have been performed by an ab initio MP2 and three density functional methods (B3LYP, M06 and M06-2X). All theoretical methods have predicted that four possible conformers of orotic acid may exist in the gas phase. The calculated barrier height for rotation of the COOH group around the CC bond (37kJmol−1, M06-2X) is much lower than the barriers for the OH rotation around the CO bond (47 and 51kJmol−1). The Gibbs free energies, relative stabilities and the mole fractions of isomers at different temperatures, in the gas phase, have been determined.The anharmonic vibrational frequencies, infrared intensities and potential energy distributions (PEDs) were computed for two isomers of the lowest energy (A and B) using the B3LYP method with the aug-cc-pVTZ basis set. The theoretical anharmonic IR spectra are in excellent agreement with the experiment. It is concluded that the most stable conformer (A) is the predominant form in a low-temperature argon matrix, while the mole fraction of the less stable B conformer can be assessed as ca. 15%. No spectral indications of the presence of other isomers (C and D) in the matrix were detected.

Rotational Isomerism of Phenylthiolated Chromophores with Large Variation of Optical Nonlinearity

We design and synthesize various π-conjugated phenyltriene chromophores having different thiolated electron donor groups such as thiol, alkylthiol, and arylthiol groups to systematically investigate the electron-donating ability of sulfur analogues. For comparison, we also investigate the commonly used electron donor groups, oxygen-containing hydroxyl and alkoxy groups, as well as nitrogen-containing dialkylamino groups. Experimental and/or theoretical analysis of the nonlinear optical properties of these chromophores in gas, solution, and solid states suggest that the rotational isomerization of an arylthiolated group acting as electron donor in polyene chromophores induces a large variation of molecular optical nonlinearities. The first hyperpolarizabilities, calculated separately with finite-field (FF) and time-dependent density functional theory (TD-DFT) two-state model methods, indicate that this phenomenon is a consequence of the competitive electronic transitions from the ground state to the first and second excited states modulated by the rotation of thiophenyl group.