Tt Conformer Research Articles

Internal Rotation in Propionic Acid: Near-Infrared-Induced Isomerization in Solid Argon

The conformational system of propionic acid (CH3CH2COOH) is studied in solid argon. It is predicted by the ab initio calculations that this molecule has four stable conformers. These four structures are denoted Tt, Tg+/-, Ct, and Cg+/-, and they differ by the arrangement around the C-O and Calpha-C bonds. The ground-state Tt conformer is the only form present at 8 K after deposition of an argon matrix containing propionic acid. For the CH3CH2COOH and CH3CH2COOD isotopologues, narrow-band excitation of the first hydroxyl stretching overtone of the conformational ground state promotes the Calpha-C and C-O internal rotations producing the Tg+/- and Ct conformers, respectively. A subsequent vibrational excitation of the produced Tg+/- form induces its conversion to the Cg+/- conformer by rotation around the C-O bond. In the dark, all of the produced conformers decay to the conformational ground state at different rates. The decay kinetics and its temperature dependence allow the identification of the conformers by IR absorption spectroscopy, which is supported by ab initio calculations of their vibrational spectra. For the CH3CH2COOD isotopologue, the excitation of molecules isolated in different matrix sites results in site-dependent photoisomerization rates for the Calpha-C and C-O internal rotations, which also confirm the identification of the photoproducts.

Infrared and Raman spectra, conformational analysis, ab initio calculations and vibrational assignment of 2-chloroethylsilyl chloride

The infrared (3100–40 cm −1) spectra of gaseous and solid and Raman (3200–20 cm −1) spectra of liquid 2-chloroethylsilyl chloride, ClCH 2CH 2SiH 2Cl, have been recorded. There are five possible stable conformers, Gg, Tg, Gt, Tt and Gg′ for this molecule where the capital letter G ( gauche) or T ( trans) refer to rotation around the C–C bond and the lower case letters to rotation around the Si–C bond. Most ab initio calculations at the MP2(full) level predicted the order of the stability as Tg>Gg>Gt>Tt>Gg′ whereas all density function theory calculations with the B3LYP method predicted the stability as Tg>Tt>Gg>Gt>Gg′. The four more stable conformers have been identified in the fluid phases with the Tg rotamer the only form remaining in the solid. Variable temperature (−105 to −150 °C) studies of the infrared spectra of the samples dissolved in liquid krypton have been recorded and the enthalpy differences determined to be: 50±20 (0.59±0.24 kJ/mol), 172±17 (2.06±0.20 kJ/mol) and 290±40 cm −1 (3.45±0.48 kJ/mol) for the Tg/Gg, Tg/Gt and Tg/Tt conformer pairs with the Tg conformer the most stable form. It is estimated that there is 42±2% of the Tg form, 33±1% of the Gg form, 20±2% of the Gt form, and 5±1% of the Tt form present at ambient temperature. A relatively complete vibrational assignment is proposed for the Tg conformer and many of the fundamentals have been identified for the other three (Gg, Gt and Tt) conformers based on the ab initio MP2(full)/6-31G(d) predicted frequencies, the relative infrared and Raman spectral intensities, and infrared band contours which are supported by normal coordinate calculations. Since the predicted energies between Tg and Gg′ conformers from all calculations are very large, it is not surprising that no evidence in either the infrared or Raman spectra could be found for the Gg′ conformer. The geometrical parameters, harmonic force constants, vibrational frequencies, infrared intensities, Raman activities, depolarization ratios, and energies have been obtained for all five conformers from ab initio MP2/6-31G(d) calculations. Structural parameters and energies have also been obtained utilizing several larger basis sets up to 6-311+G(3df,2pd). The results from these vibrational and theoretical studies are discussed and compared to those obtained for some similar molecules.

Matrix-isolation FT-IR spectra and theoretical study of dimethyl sulfate

The preferred conformations of dimethyl sulfate and their vibrational spectra were studied by matrix-isolation FT-IR spectroscopy and theoretical methods (DFT and MP2, with basis sets of different sizes, including the quadruple-zeta, aug-cc-pVQZ basis). Conformer GG (of C 2 symmetry and exhibiting O S O C dihedral angles of 74.3°) was found to be the most stable conformer in both the gaseous phase and isolated in argon. Upon annealing of the matrix, the less stable observed conformer (GT; with C 1 symmetry) quickly converts to the GG conformer, with the resulting species being embedded in a matrix-cage which corresponds to the most stable matrix-site for GG form. The highest energy TT conformer, which was assumed to be the most stable conformer in previous studies, is predicted by the calculations to have a relative energy of ca. 10 kJ mol −1 and was not observed in the spectra of the matrix-isolated compound.

Matrix isolation FT-IR, FT-Raman spectroscopy, conformational ab initio calculations, and vibrational frequencies of meso and racemic-2,4-pentanediol

Abstract The infrared spectra of meso-2,4-pentanediol and racemic-2,4-pentanediol were measured in an argon matrix at 20 K. The Raman spectra of the pure liquids (meso and racemic) were measured at room temperature. The spectra were obtained using a Fourier transform spectrophotometer and a cryostat for the low temperature matrix. The meso and racemic forms of the diol were separated by means of a spinning band distillation column. The energies of nine possible conformers of the meso form and nine conformers of the racemic form were calculated. Extensive ab initio calculations using B3LYP, MP2 and HF methods with several basis sets consistently gave the lowest energy for the TT conformer of the meso form and the G − T (= TG − ) conformer of the racemic form. Ab initio calculations at the B3LYP/6-31G** level were performed for the lowest energy conformer of meso and racemic pentanediol to obtain the equilibrium geometry, vibrational frequencies, and infrared and Raman intensities. Calculated and experimental frequencies were compared to make vibrational assignments.

The rotational spectra of single molecular eigenstates of 2-fluoroethanol: Measurement of the conformational isomerization rate at 2980 cm−1

The rotational spectroscopy of single molecular eigenstates has been used to measure the conformational isomerization rate in 2-fluoroethanol. Eigenstates in the asymmetric –CH2(F) stretch spectrum of the Gg′ conformer near 2980 cm−1 are prepared with an infrared laser. These eigenstates are approximately 2000 cm−1 above the barrier to Gg′−Tt conformational isomerization. The rotational spectrum is measured using an infrared-microwave double-resonance technique based on the Autler–Townes splitting of states in a strong microwave field. This technique does not require saturation of the infrared preparation step. Two types of rotational transitions are observed. These are assigned to rotational transitions from vibrational states with Tt conformation (near 15.8 GHz) and to “isomerization states” (near 17.1 GHz) where the torsional wave functions are above the isomerization barrier. The isomerization kinetics are obtained from the linewidth of the ensemble eigenstate rotational spectrum. The lifetime for the Tt conformer is 2.7 ns. The isomerization states relax at approximately twice the rate of the Tt states (1.5 ns lifetime). This result is consistent with a kinetics model where the isomerization proceeds by “over-the-barrier” pathways. Both lifetimes are longer than the bright-state IVR lifetime (275 ps) indicating that the intramolecular dynamics occur on two distinct time scales. The isomerization rate for the Tt states is three orders-of-magnitude slower than predicted by a simple RRKM rate expression.

Intramolecular vibrational energy redistribution and conformational isomerization in vibrationally excited 2-fluoroethanol: High-resolution, microwave-infrared double-resonance spectroscopy investigation of the asymmetric –CH2(F) stretch near 2980 cm−1

The asymmetric –CH2(F) stretch spectrum of 2-fluoroethanol near 2980 cm−1 has been rotationally assigned using microwave-infrared double-resonance spectroscopy methods in an electric-resonance optothermal molecular-beam spectrometer. The eigenstate-resolved infrared spectrum shows the effects of intramolecular vibrational energy redistribution (IVR) through the fragmentation of each rotational level of the vibrationally excited state into a set of transitions. From the spectrum we determine the IVR lifetime of the asymmetric –CH2(F) stretch to be 275 ps. The measured vibrational state density at 2980 cm−1 is 44 states/cm−1, and matches the value for the total state density obtained from a direct count. This agreement suggests that vibrational states of both the Gg′ and Tt conformers are coupled by the intramolecular dynamics. From measurements of the c-type pure rotational transitions of the Gg′ conformer we determine that the tunneling splitting for the Gg′ ground state is less than 35 kHz. The infrared spectrum is characterized by a large number of closely spaced infrared transitions. The clustering of vibrational energy levels is attributed to the weak interaction between the degenerate Gg′ and nondegenerate Tt vibrational states. This lifting of the effective Gg′ parity degeneracy is quantitatively investigated through the shape of the nearest-neighbor level spacing distribution. From this analysis we estimate that the isomerization lifetime for the Gg′ conformer of 2-fluoroethanol is 2 ns.

Spectroscopic evidence for an intramolecular C–H⋯N hydrogen bond: infrared and Raman spectroscopy and ab initio molecular orbital calculations of N, N-dimethylpropylamine and propyltrimethylammonium bromide

The infrared and Raman spectra were measured for N, N-dimethylpropylamine (DPA) in an argon matrix, the liquid and glassy states and aqueous solution, and for propyltrimethylammonium bromide (PTAB) in the solid state and aqueous solution. The ab initio molecular orbital calculations were performed on possible conformers of DPA and propyltrimethylammonium ion. In an argon matrix, the conformational stability of DPA is in the order G′G>GT>TT, which agrees with the prediction by ab initio calculations. The experimentally confirmed high stability of the G′G conformer indicates that an appreciably strong attractive interaction occurs between the methyl hydrogen atom and the nitrogen atom. This interaction may be called the intramolecular 1,4-C–H⋯N hydrogen bonding. In the liquid and glassy states, the GT conformer is the most stable. In aqueous solution of DPA, the GT and TT conformers are more stabilized than in the neat liquid, possibly because of the formation of more significant intermolecular hydrogen bonds between the nitrogen atom of DPA and water molecules. For PTAB, only the T conformer exists in the solid state, and this conformer is more stable than the G conformer in aqueous solution. The GG and TG conformers of DPA and the G conformer of PTAB are significantly less stabilized by steric repulsions between the CH 3(N) and (C)CH 3 groups.

Structurally mixed molecular eigenstates of 2-fluoroethanol resulting from conformational isomerization

Using a newly developed method of molecular-beam, infrared-microwave double-resonance spectroscopy, we are able to measure the rotational spectrum of a single molecular eigenstate of the molecular Hamiltonian near 3000 cm−1 of energy above the ground rovibrational state. This energy lies above the barrier to conformation isomerization in many molecules. In the –CH2F asymmetric C–H stretch of the Gg conformer of 2-fluoroethanol, near 2983 cm−1, we demonstrate the contribution of vibrational states localized around the Tt conformer structural minimum to the individual molecular eigenstates. The measurement demonstrates the ability of isolated molecules to use vibrational excitation to achieve geometrical rearrangement.

Gas-phase structure and conformation of diethyl ketone studied by molecular orbital constrained electron diffraction

The molecular structure and conformation of diethyl ketone (3-pentanone), C(3)H 3C(2)H 2C(1)OC(4)H 2C(5)H 3, at 27°C were studied by gas electron diffraction (GED). Vibrational mean amplitudes and shrinkage corrections were calculated on the basis of the vibrational spectra measured in the vapor and liquid phases. Structural constraints were obtained from RHF/6-31G∗ ab initio calculations. Diffraction data were reproduced best by a mixture of two conformers, trans-trans (TT, dihedral angles ф 1(C 4C 1C 2C 3) = ф 2(C 2C 1C 4C 5) = 180°) and trans-gauche (TG). The structural parameters of the TT form determined by GED, with error estimates (3σ) in parentheses, are: r g (CO) = 1.217(2)Å, r g (C 1−C) = 1.519(1)Å, r g (C 2−C 3) = 1.524(1)Å, 〈 r g (C−H〉 = 1.103(3)Å, ∠ α CC(O)C = 116.0(6)°, ∠ α C(O)CC = 114.2(6)°, ∠ α C(O)CH = 108.2(7)°, and 〈∠ α C 2C 3H〉 = 111.4(7)°, where 〈 〉 denotes average value. The difference between r(C 1−C) and r(C 2−C 3) was assumed. Similarly ∠C(O)CH and ∠C 2C 3H were refined in groups. The structural differences between the TT and TG conformers were taken from the 6–31G∗ calculations except for the dihedral angles. The dihedral angles of the TG form were determined to be ф 1 = 61(7)° and ф 2 = 151(14)° . The relative abundances of the TT and TG conformers are 50(10) and 50%, respectively.

Gas-phase structure and conformation of diethyl ketone studied by molecular orbital constrained electron diffraction

The molecular structure and conformation of diethyl ketone (3-pentanone), C(3)H 3C(2)H 2C(1)OC(4)H 2C(5)H 3, at 27°C were studied by gas electron diffraction (GED). Vibrational mean amplitudes and shrinkage corrections were calculated on the basis of the vibrational spectra measured in the vapor and liquid phases. Structural constraints were obtained from RHF/6-31G∗ ab initio calculations. Diffraction data were reproduced best by a mixture of two conformers, trans-trans (TT, dihedral angles F 1(C 4C 1C 2C 3) = F 2(C 2C 1C 4C 5) = 180°) and trans-gauche (TG). The structural parameters of the TT form determined by GED, with error estimates (3σ) in parentheses, are: r g( CO) = 1.217(2) A ̊ , r g( C 1C ) = 1.519(1) A ̊ , r g( C 2C 3 ) = 1.524(1) A ̊ , 〈r g( CH)〉 = 1.103(3) A ̊ , ∠ α CC(O)C = 116.0(6)°, ∠ α C(O)CC = 114.2(6)°, ∠ α C(O)CH = 108.2(7)°, and 〈∠ α C 2C 3H 〉 = 111.4(7)° , where 〈 〉 denotes average value. The difference between r (C 1C) and r(C 2 3) was assumed. Similarly ∠C(O)CH and ∠C 2C 3H were refined in groups. The structural differences between the TT and TG conformers were taken from the 6–31G∗ calculations except for the dihedral angles. The dihedral angles of the TG form were determined to be F 1 = 61(7)° and F 2 = 151(14)°. The relative abundances of the TT and TG conformers are 50(10) and 50%, respectively.

Reexamination of the conformations of dimethyl phosphate anion in water

Monte Carlo simulations were employed to calculate the relative free energies of hydration for the gauche-gauche (gg), gauche-trans (gt), and trans-trans (tt) conformers of the dimethyl phosphate anion. Relative to the gg conformer, the free energy of hydration of the gt conformer is 0.2 kcal mol −1 higher, while that of the tt conformer is 2.6 kcal mol −1 lower. In combination with the free energy differences for the isolated molecules (Liang et al., J. Am. Chem. Soc., 115 (1993) 1537), these results indicate that in aqueous solution the gg conformer is most stable, but the gt and tt conformers have comparable stabilities. The entropic contribution to the free energy of hydration is thought to be an important factor.

Structural and conformational analysis of diethyl ether by molecular orbital constrained electron diffraction combined with microwave spectroscopic data

The molecular structure and conformation of diethyl ether, C(3)H 3C(2)H 2O(1)C(4)H 2C(5)H 3, at 27°C was studied by a joint analysis of gas electron diffraction and microwave spectroscopic data. Vibrational mean amplitudes and shrinkage corrections were calculated on the basis of the vibrational spectra measured in the vapor and liquid phases. Ab initio geometry optimization at the HF/4-21G level was performed on four possible conformers and the results were used as structural constraints. Diffraction and vibrational spectroscopic data are consistent with the presence of two conformers, trans-trans (TT, dihedral angles φ 1(C 4O 1C 2C 3) = φ 2(C 2O 1C 4C 5) = 180°) and trans-gauche (TG), in the vapor phase. The compositions of the TT and TG conformers were determined to be 69(8) and 31%, respectively, by the joint analysis. The structural parameters of the TT form determined are r g(OC) = 1.419(1) Å, r g(CC) = 1.514(2) Å, 〈 r g(CH)〉 = 1.114(2) Å, ∠ αCOC = 113.5(4)°, ∠ αOCC = 108.6(1)°, ∠ αOCH = 110.4(9)°, and 〈∠ αCCH〉 = 110.5(7)°, where values in parentheses are error estimates (3σ) and those in angle brackets denote average values. The structural differences between the TT and TG conformers were taken from the 4-21G calculations except for the dihedral angles of the TG form, which were determined to be φ 1 = 76(10)° and φ 2 = 161(15)°.

An 1H NMR conformational analysis of 3-phenylpentane in solution

Some 30 proton chemical shifts and proton–proton coupling constants are reported for a 4.7 mol% solution of 3-phenylpentane in a CS2/C6D12/TMS solvent mixture at 300 K. The long-range coupling constant over six formal bonds between the methine and para protons is used to deduce an apparent twofold barrier of 15.0 ± 0.3 kJ/mol to rotation about the Csp2—Csp3 bond, at least twice as large as that for isopropylbenzene in solution. AM1 computations agree with experiment in finding the conformation of lowest energy as that in which the methine C—H bond is situated in the phenyl plane, but predict a barrier height of only 13.9 kJ/mol. The vicinal coupling constants are consistent with a fractional population, 0.38(2), of the TT conformer, that in which all the carbon atoms of the alkyl chain lie in a plane. A doubly degenerate conformer, TG+(G−T), in which one methyl group is twisted away from the phenyl substituent, then has a fractional population of 0.62(2). The assumption that only these three conformers are present is tested with the signs and magnitudes of the four different coupling constants over four bonds. These coupling constants are consistent with the absence of significant proportions of the other six all-staggered conformers. These six are characterized by a close approach of the methyl groups (1,5 interactions) or by proximity of the methyl and phenyl moieties.

Structural and conformational analysis of N-chloro- N-ethylethanamine by gas electron diffraction, ab initio calculations and vibrational spectroscopy

The structural and conformational properties of N-chloro- N-ethylethanamine, C(3)H 3C(2)H 2N(1)ClC(4)H 2C(5)H 3, were studied by gas electron diffraction (GED). Vibrational amplitudes and shrinkage corrections were calculated on the basis of a normal coordinate analysis of the vibrational spectra measured in the vapor and liquid phases. The geometries of six possible conformers were optimized by ab initio HF/4-21G */3-3-21G * calculations and the results were used in the data analysis as structural constraints. At 25°C, two conformers were observed. On the basis of the GED data analysis it is possible to identify the trans-trans conformer (TT, dihedral angles φ 1(C 4N 1C 2C 3)= −φ 2(C 2N 1C 4C 5) = 175(1)°) as the most stable one, but the nature of the minor component is uncertain, since various mixtures of TT with one of three different gauche forms, TG, TG′ or GG′, fit the experimental data reasonably well. The best fitting model is a mixture of TT and TG (dihedral angles φ 1 = 80(6)° and φ 2 = −175(1)°), with a relative abundance of 68(8) and 32%, respectively. The vibrational spectra are consistent with the existence of two conformers in the vapor and liquid phases. The structural parameters of TT determined by this analysis (with error limits of 3σ and average values in brackets) are r g(NC) = 1.471(2) Å, r g(NCl) = 1.765(2) Å, r g(CC) = 1.530(3) Å, 〈 r g(CH)〉 = 1.118(3) Å, ∠ α NCC = 112.6(4)°, 〈 ∠ α NCH> = 109.6(18)°, ∠ α CNC = 11066(8)°, ∠ α CNCl = 108.3(2)°, < ∠ α CCH > = 109.4(11)° and φ 1 = (−φ 2) = 175(1)°. The structural differences between the TT and TG conformers were taken from the calculated ones.

Resonance Raman and absorption spectroscopy of 2- and 3-methyl-1,3,5-hexatriene in the T1 state: steric control of excited-state molecular structure

Resonance Raman (RR) spectra of (E)-2-methyl-1,3,5-hexatriene [(E)-2-MHT], (E)-4-deuterio-2-methyl-1,3,5-hexatriene, (Z)-2-methyl-1,3,5-hexatriene [(Z)-2-MHT], (E)-3-methyl-1,3,5-hexatriene [(E)-3-MHT], and (Z)-3-methyl-1,3,5-hexatriene [(Z)-3-MHT] in the lowest triplet state T1 are reported and discussed. QCFF/PI calculations are performed to determine energies and optimized geometries in the S0, T1, and Tn electronic states. For each T1 species, T1 AE Tn transitions, vibrational frequencies, and RR intensities are calculated and compared with the observed frequencies and intensities. The ground-state distribution of rotamers is shown to be preserved upon excitation to T1 and during the T1 lifetime (NEER principle). In contrast, the E and Z isomers associated with the torsion around the central CC bond are found to equilibrate in T1 according to the Boltzmann rule. In 2-MHT, the T1 RR spectrum is attributed to both the E and Z forms, the former being dominant. For (E)-2-MHT, only the tt conformer is shown to be present, while for (Z)-2-MHT also the contribution of the ct conformer is demonstrated. The T1 RR spectrum of 3-MHT receives a larger contribution from the Z than from the E isomer. The spectra obtained from (E)-3-MHT and (Z)-3-MHT are identical, since in both isomers only the tt conformer is present. Measurements of time-resolved triplet-triplet absorption as a function of temperature yield activation energies and frequency factors for the decay of the T1 state of (E)-2-MHT and (Z)-3-MHT. The implications of these results on the shape of the T1 potential energy curves are discussed.

High pressure study on molecular conformational equilibria of n-pentane

The n-pentane molecule has the basic conformations, TT, TG, and GG. It is important for understanding the conformational behavior of chain molecules. We have studied the effect of pressure on the conformations of n-pentane in the condensed phase by using high pressure infrared spectroscopy with a diamond anvil cell up to 28.9 kbar. Compression of liquid n-pentane increases the concentration of the more globular conformers, while the TT conformer only exists as a high pressure solid. The volume changes for TT to TG, TG to GG, and TT to GG forms are −1.0±0.2, −1.1±0.3, and −2.1±0.3 cm3/mol, at 20 °C, respectively. The conformational volume changes in liquid n-alkanes are discussed in terms of the solvent-excluded volume.

Molecular structure and glass temperature in surface layers of poly(methyl methacrylate) films. IR spectroscopic study

By the methods of IR spectroscopy, the characteristics of molecular structure and the glass temperature T g were determined in the surface layers of PMMA films. It was found that near to the interphase boundary, the PMMA macromolecules exhibit an enhanced content of the high-energy tg-conformers, at the expense of the energetically more favoured tt conformers. The concentration of tg-conformers increases monotonously with decreasing thickness d of the polymer film, starting from d = 10 μm. Moreover, at d < 4 μm, the interactions between side chains are enhanced. T g was found to change in a complex (nonmonotonous) manner with decreasing film thickness, starting from d = 10 μm decreases, attaining a minimum at d = 3.5 μm, and begins to grow further on. The observed behaviour of T g could be fully explained in terms of the Gibbs-DiMarzio theory, relating T g to the rotational-isomeric characteristics of the polymer chain.

Vibrational selectivity in the single-photon infrared photochemistry of matrix-isolated 2-fluoroethanol

Relative quantum efficiencies for the IR-induced G′g to Tt conformer interconversion of 2-fluoroethanol in solid argon are reported. Laser excitation was used to investigate ten vibrational bands of 2-fluoroethanol with frequencies between 952 and 7121 cm−1. Three bands of the deuterated (OD) analog were also investigated using laser and filtered glower excitation. Deviations of the quantum efficiencies from a smooth, monotonic function of energy were significantly larger than the experimental uncertainties. Anomalously high quantum efficiencies were found for modes with strong coupling to the COH torsion.

Temperature- and solvent-dependent raman bands of n-propylamine-N-d 2: a conformational study

Temperature-dependent intensities of Raman bands of n-propylamine-N-d 2 confirm previous normal coordinate assignments [1] and yield a trans-gauche energy difference of 1.7 kJ mol −1 in close agreement with previous ab initio MO calculations of n-propylamine [1]. In addition, the bands ascribed to the TT conformer are favoured by reducing the temperature or increasing the polarity of the solvent.

Infrared-induced conformational changes of 2-halogenoethanols in low-temperature matrices

The infrared-induced isomerisation of 2-chloroethanol, 2-bromoethanol and 2-iodo-ethanol was studied in argon, nitrogen and methane matrices. Isomerisation was observed to occur only in argon matrices and was about three times slower for 2-bromoethanol than for 2-chloroethanol, whilst it was too slow to measure for 2-iodoethanol under the conditions used. The complex multiplet features observed in the CX stretching region have been interpreted in terms of isomerisation from the predominant Gg′ conformer to a mixture of Tt and Tg conformers in highly dilute matrices, while in concentrated matrices distinct bands due to multimers of the Tt or Tg conformer were identified.