Free Internal Rotation Research Articles

Infrared and Raman spectra, conformational stability, ab initio calculations, and vibrational assignment of 5-chloropent-2-yne

The infrared (3400–50 cm −1) and/or Raman (3400–10 cm −1) spectra of gaseous, xenon solution, liquid and solid 5-chloropent-2-yne, CH 2ClCH 2CCCH 3, have been recorded. These data indicate that the molecule exists in the anti (the C–Cl bond is trans to the CC bond) and the gauche conformations in the vapor and liquid but only the anti conformer remains in the solid state. From a variable temperature infrared study of the xenon solution, the anti conformation has been determined to be more stable than the gauche form by 233±23 cm −1 (2.79±0.28 kJ/mol) and it is estimated that 39% of the sample is in the gauche form at ambient temperature. The optimized geometries, conformation stabilities, harmonic force fields, Raman activities, depolarization ratios, and infrared intensities have been obtained from ab initio MP2/6-31G(d) calculations with full electron correlation. These predicted quantities are compared to the corresponding experimental quantities when appropriate. Equilibrium geometries and energies for both conformers have been obtained from ab initio MP2/6-311G(d,p), MP2/6-311G(2d,2p) and MP2/6-311G(2df,2pd) calculations. Vibrational assignments for the 24 normal modes for the anti conformer are proposed and several of the fundamentals for the gauche conformer are assigned. The sub-band structure on the pseudo-degenerate vibrations of the methyl group indicates that it is almost free internal rotation. From this fine structure, the Coriolis coupling constants, ξ, have been determined. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.

Simulation of short-chain polymer collapse with an explicit solvent

We study the equilibrium behavior and dynamics of a polymer collapse transition for a system composed of a short Lennard-Jones (LJ) chain immersed in a LJ solvent for solvent densities in the range of ρ=0.6–0.9 (in LJ reduced units). The monomer hydrophobicity is quantified by a parameter λ∈[0,1] which gives a measure of the strength of attraction between the monomers and solvent particles, and which is given by λ=0 for a purely repulsive interaction and λ=1 for a standard LJ interaction. A transition from the Flory coil to a molten globule is induced by increasing λ. Generally, the polymer size decreases with increasing solvent density for all λ. Polymer collapse is induced by changing the hydrophobicity parameter from λ=0 to λ⩾0.5, where the polymer is in a molten globule state. The collapse rate increases monotonically with increasing hydrophobicity and decreases monotonically with increasing solvent density. Doubling the length of the chain from N=20 to N=40 monomers increases the collapse time roughly by a factor of 2, more or less independent of the hydrophobicity and solvent density. We also study the effect of conformational restrictions on polymer collapse using a chain model in which the bond angles are held near 109.5° using a stiff angular harmonic potential, but where free internal rotation is allowed, and find that the collapse times increase considerably with respect to the fully flexible polymer, roughly by a factor of 1.6–3.5. This increase is most pronounced for high solvent densities.

Ab initio Calculations of the Internal Rotation Barrier in the Dimethylacetylene Molecule

Ab initio quantum-chemical calculations have been successfully used to study the torsional potentials of molecules (for example, [1]). At the same time, there are few calculations for compounds whose internal rotation barriers are less than kT . This is due to the fact that a comparison of the total energies of conformations with a small energy difference is insufficiently reliable. Recently, there have been a number of papers devoted to both quantum-chemical and experimental investigations of the torsional potentials of molecules with practically free internal rotation [2, 3]. The goal of the present study is to estimate the potentials of ab initio quantum-chemical calculations of internal rotation barriers well below kT . We studied dimethylacetylene (DMA), which has one of the lowest rotation barriers known in organic chemistry. DMA is a classical example of practically free relative internal rotation of methyl groups [4]. The rotation barrier for this compound is rather low, and attempts at determining its value have long been unsuccessful [5–7]. The rotation barrier for the methyl groups in DMA was first determined in 1971 [8]. According to the results of this work, it is ∼ 48 J/mole. Later, owing to the development of spectral techniques, the value of the rotation barrier in DMA was established more reliably. According to the data based on interpretation of a vibrational spectrum of DMA in the region 1000–1100 cm−1 taken with a resolution of 0.0014 cm−1, the internal rotation barrier is 62.2 ± 1.0 J/mole [9, 10]. In addition, in the papers cited, it is noted that the position of the minimum of the torsional potential cannot be determined by IR spectroscopy. In another study, an analysis of an IR spectrum of DMA recorded in the range 2955–3065 cm−1 with a resolution of 0.0028 cm−1 gave a value of 75.60± 0.41 J/mole for the rotation barrier [11]. In the first ab initio quantum-chemical investigations, the internal rotation of the methyl groups of DMA was ignored. In papers describing calculations for DMA, the type of conformation (eclipsed or skewed) considered is not indicated and the rotation of the substituent around the threefold axis of the acetylene bond is ignored [12, 13]. In [14], the difference in energy between the DMA conformations is estimated in ab initio calculations. For the skew conformation, the value of the barrier calculated by the HF/4-31G method is 30 kJ/mole. In the present work, we calculated the energy difference between the skew (D3d) and eclipsed (D3h) conformations of DMA. The GAUSSIAN-94 software package was used [15]. The geometry was optimized using the HF, BLYP, B3LYP, and MP2 methods and sets of basis functions from 6-31G(d) to 6-311G++(3df , 3p). Calculations of the conformers with a fixed torsion angle were carried out with full optimization of the remaining geometrical parameters. The energies of the conformers were refined by the MP4 method, and their structures were optimized by the MP2 method. The calculations showed that the eclipsed conformer of symmetry D3h is more stable. This agrees with the conclusion of [14]. The difference in bond lengths between the examined structures of the DMA molecule is extremely small and does not exceed 0.004–0.006 pm for different methods. The variations in the electronic structure of DMA for the values of the torsion angle considered are also inappreciable.

The structure and spectra of 1-silylpropyne: scaled quantum-chemical force fields and vibrational effects

The geometric parameters and the force fields of two 1-silylpropyne conformers (eclipsed and staggered, both of C3v symmetry) were calculated by the restricted Hartree–Fock method and taking into account electron correlation in the approximation of Møller–Plesset second-order perturbation theory (6-311G∗∗ basis set). The total energies of the two conformers differ by less than 20calmol−1, which is evidence of virtually free internal rotation. The IR spectra of H3SiCCCH3 and D3SiCCCH3 were interpreted by scaling quantum-chemical force fields. The scale factors were found to be transferable in the series of Si and Sn acetylenides. The mean vibrational amplitudes and the harmonic corrections to internuclear distances were calculated taking into account nonlinear kinematic effects. Corrections for centrifugal distortions and anharmonic effects were calculated at the level of first-order perturbation theory. The introduction of these corrections into the experimental rij,a parameters allowed us to determine the parameters of the rh1 structure and estimate equilibrium internuclear distances.

Molecular and Electronic Structure of C5H5N−SO3: Correlation of Ground State Physical Properties with Orbital Energy Gaps in Partially Bound Lewis Acid−Base Complexes

The donor−acceptor complex C5H5N−SO3 has been studied in the gas phase using rotational spectroscopy. The adduct has the expected geometry in which the nitrogen lone pair is directed along the C3 axis of the SO3 and the SO3 undergoes free or nearly free internal rotation within the complex. The N−S bond length is 1.915(1) A, and the NSO angle is 98.91(2)o, indicating that the formation of the dative bond is nearly, but not entirely, complete. Small but significant changes in the heavy-atom ring structure of the pyridine upon complexation are also measured by a series of 13C substitution experiments. Analysis of the 14N quadrupole coupling constants indicates a transfer of approximately 0.54 electrons away from the pyridine upon formation of the dative bond. In the series of complexes of SO3 with HCN, CH3CN, H3N, C5H5N, and (CH3)3N, electron-transfer values increase as the bond length decreases, and these changes are shown to accompany a gradual decrease in the energy gap between the lone pair orbital of t...

Calculation of rovibrational states of weakly bound complexes by transformation from an Eckart frame: Benzene-N2

We apply a method for the construction of rotational-intermolecular vibrational states to the van der Waals molecule benzene-N2. The method, as described in Felker et al. [J. Chem. Phys. 114, 1233 (2001)] is based on the transformation of approximate rovibrational eigenfunctions from functions of coordinates referred to an Eckart body-fixed axis system to functions of coordinates referred to a more convenient body-fixed frame. This transformation allows the computation of approximate rovibrational eigenenergies, as well as other molecular properties, from one or a small set of J=0 eigenfunctions. By comparing rovibrational energies and matrix elements computed by this Eckart method with those computed variationally with a large basis, we show that the former yields excellent results for benzene-N2. The results are particularly significant because of the presence of essentially free internal rotation in the species, a large-amplitude motion that complicates the definition of an Eckart frame.

Low-temperature dynamics and spectroscopy in exohedral rare-gas C60 fullerene complexes

The adatom dynamics in exohedral C60 fullerene complexes of rare-gas atoms are studied with a three degrees of freedom model. The eigenvalue problem of the corresponding quantum Hamiltonian is solved and the electric-dipole spectra for ArC60, NeC60, and HeC60 in the low-temperature range from 5 to 40 K are simulated. The most important spectral features are related to the degree of angular anisotropy in the adatom–C60 interaction. The ArC60 and NeC60 complexes present very simple spectra which can be assigned in terms of three-mode oscillators; the corresponding motion takes place in the deep hexagon wells (also in the pentagon wells for NeC60) of the interaction potential. On the contrary, the HeC60 complex shows more complicated spectra with important tunneling effects due to the smaller angular anisotropy of the interaction. The onset of almost free internal rotation takes place in this complex at rather low energies, and this gives rise to a low-frequency rotational band in the spectra at temperatures above T∼15 K.

Structure, spectra, and intramolecular motions of 1,4-bis(trimethylsilyl)-1,3-butadiyne: an electron diffraction study using the results of spectral analysis based on scaled quantum-chemical force field for calculating vibrational effects

The molecular structure of 1,4-bis(trimethylsilyl)-1,3-butadiyne has been studied by the gas electron diffraction method and by ab initio calculations at the RHF and MP2 levels. A scaled quantum-chemical force field was used to interpret the vibrational spectra. The root-mean-square vibrational amplitudes and harmonic shrinkage corrections were calculated taking into account non-linear relations between Cartesian and internal vibrational co-ordinates at the level of first-order perturbation theory (h1) and with the use of the traditional scheme (h0). The total energies of the D3d and D3h conformers differ by about 1calmol−1 (0.4cm−1) which implies virtually free internal rotation. It has been shown that the flexibility of the linear fragment in (CH3)3Si–CC–CC–Si(CH3)3 has increased considerably compared to that in (CH3)3Si–CC–Si(CH3)3.

Intermolecular potential energy surface of the proton-bound H2O+–He dimer: Ab initio calculations and IR spectra of the O–H stretch vibrations

Rotationally resolved infrared photodissociation spectra of the O–H stretch fundamentals (ν1 and ν3) of the H2O+–He open shell ionic complex have been recorded in the doublet electronic ground state. The analysis of the complexation-induced frequency shifts (Δν1 = − 15 cm−1, Δν3 = − 5.1 cm−1) and the rotational structure is consistent with a planar, translinear proton-bound H–O–H–He equilibrium geometry. The derived intermolecular H–He separation and O–H–He bond angle are R0 = 1.756(4) A and φ0 = 175(5)° in the ground vibrational state. The experimental results are in good agreement with the ab initio calculations performed at the unrestricted MP2 level using a basis set of aug-cc-pVTZ quality. The global minimum on the calculated potential energy surface features a slightly translinear proton-bound equilibrium geometry, with an intermolecular separation of Re = 1.6990 A, a bond angle of φe = 173.2°, dissociation energies of De = 425.9 cm−1 and D0 = 158.6 cm−1, and frequency shifts of Δν1 = − 34.4 cm−1 and Δν3 = − 11.6 cm−1. Tunneling splittings in the perpendicular component of the ν3 band are attributed to hindered internal rotation exchanging the two equivalent proton-bound structures ia a planar transition state with C2v symmetry. The calculated barrier for this internal motion amounts to Vb = 202.9 cm−1 and the dimer is closer to the semirigid limit than to free internal rotation. The interpretation of the H2O+–He spectrum is supported by the corresponding spectrum of the monodeuterated HOD+–He complex.

Numerical Evaluation of Energy Levels and Wave Functions for Hindered Internal Rotation

A numerical procedure for the solution of the one-dimensional Schrödinger equation (the finite-difference boundary-value method), especially suited for problems having periodic nature, is presented and applied to the problem of hindered internal rotation. The accuracy of the numerical method, as a function of the number of mesh points, is evaluated by comparing the approximate numerical results with the exact ones, relative to the case of free internal rotation. The hindered rotation in ethane is then examined in detail, and the character of the obtained energy levels is discussed by a comparison with the energy levels of the two limiting cases of torsional vibration and shifted potential free rotation. An approximate procedure for the treatment of internal rotations involving asymmetric tops is also suggested, and applied to the case of 1,2-dichloroethane. The results show a satisfactory agreement with those obtained by the more elaborate method of Chung-Phillips and with the experimental ones.

The à 2Σ+ state of Ar⋅NO

The à 2Σ+ state of Ar⋅NO is studied using (1+1) resonance-enhanced multiphoton ionization (REMPI) spectroscopy. Higher quality spectra than obtained in other studies allow the identification of a number of previously unreported features. The spectrum is analyzed using two models: a rigid van der Waals complex in which NO is weakly bonded to Ar; and a complex in which the free internal rotation of NO is hindered by the anisotropy caused by the presence of the Ar atom. It is concluded that as the intermolecular stretch is excited, then the anisotropy decreases, and the angular motion of the complex becomes more and more like that of a free rotor. Near the origin, the complex has an average geometry approaching linear, whereas when the intermolecular stretch is excited, an average geometry closer to T-shaped occurs; however, when the anisotropy is small, the concept of geometry becomes ill-defined.

The structure of O3–CH4 and the implications for the O+CH4 precursor-initiated reaction

The rotational spectrum of the O3–CH4 complex has been measured in a molecular beam using a pulsed-nozzle Fourier-transform microwave spectrometer. An a-type pure-rotation and a c-type rotation-inversion electric-dipole spectrum is observed, complicated by the nearly free internal rotation of the CH4 top and the inversion tunneling of the O3. The nuclear-spin statistics of the equivalent oxygen nuclei leads to only one tunneling component existing for each rotation–internal-rotation state, indicating that the transition state has a heavy-atom, C2v-symmetry geometry. The tunneling splitting is determined to be 30 to 40 MHz, dependent on the CH4 internal-rotor state. Only two of the three methane internal-rotor states have been assigned. These two states of A and F symmetry have asymmetric-rotor energy-level structures, weakly perturbed by the ozone-inversion tunneling. The zero-point structure of the complex has a heavy-atom plane of symmetry with the two terminal O atoms equidistant above and below this plane. The angle between the line joining the center of masses of the two subunits and the O3, C2 axis is 118.2(5)°, with the central O directed away from the CH4. The shortest O–C separation is 3.57 Å. The geometry of the complex suggests two outcomes for the reaction of an O atom produced by 267 nm photolysis of O3 in the complex (assuming that the initial O3 photodissociation dynamics are not perturbed by complexation), either nonreaction or reaction by stripping of a hydrogen atom at high impact parameters, leading to fast, highly rotationally excited, OH product.

Molecular structures of acetylene derivatives of tin

The electron diffraction data on bis(trimethylstannyl)acetylene, Me3SnC≡CSnMe3, were analyzed in the framework of the one-dimensional dynamic model of free internal rotation of the SnMe3 group about the axis of the Sn−C≡C−Sn linear fragment. The root-mean-square amplitudes and harmonic shrinkage corrections used in the analysis were calculated from the scaled quantum-chemical force field (i) taking into account nonlinear relations between Cartesian and internal vibrational coordinates at the first-order level of perturbation theory (h1) and (ii) using a conventional approach (h0). Therh1 parameters of internuclear distances describe the equilibrium geometry of the Me3SnC≡CSnMe3 molecule much better than the commonly accepted parametersrα≡rh0. Substituent effects on the geometry of the acetylene fragment are discussed.

Structure and vibrational dynamics of the benzene dimer

Point-wise evaluated coupled-cluster single double triple [CCSD(T)] stabilization energies are used to parameterize the nonempirical model (NEMO) empirical intermolecular potential of the benzene dimer in the ground electronic state. The potential is used for theoretical interpretation of the dimer structure and the dynamics of its intermolecular motions. Only one energy minimum, corresponding to the T-shaped structure, is found. A parallel displaced structure is the first-order transition structure separating the molecular symmetrically equivalent T-shaped structures. Due to a relatively high transition barrier (∼170 cm−1), the interconversion tunneling is unimportant in the energy region spanned by the available rotational spectra and is thus neglected (accordingly, the molecular symmetry group which is used for interpretation of the available experimental spectra is related to the T-shaped structure with two feasible internal rotations and nonequivalent monomers). The dimer undergoes a nearly free internal rotation along the axis connecting the benzene centers of mass in the T-shaped equilibrium geometry and a hindered internal rotation (the barrier being ∼46 cm−1) along the axis that is perpendicular to the “nearly free” internal rotation axis. The tunneling splittings observed in the rotational spectrum are likely due to this hindered rotation. An analysis assuming the latter rotation as an independent motion and using purely vibrational tunneling splittings (obtained by extrapolating to zero values of the rotational quantum numbers) indicates that the genuine value of the hindered rotation barrier is nearly twice higher than its ab initio value. Similarly, the difference ΔR=0.25 Å between the ab initio (equilibrium) and experimental (ground state) values for the distance of the mass centers of the benzene monomers is strong evidence that our theoretical potential is much shallower than the genuine one. The Raman bands observed at the 3–10 cm−1 region seem to involve states associated with the nearly free rotation and the “energy minimum path” bending motion. Unambiguous assigning of the weaker Raman features is infeasible, partly due to limitations in the accuracy of the theoretical potential, and partly due to the lack of knowledge of the polarizability tensor of the dimer and temperature at which the spectra were taken.

Bis(chloromethylsilyl)amine and bis(chloromethylsilyl)methylamine; preparation, reactivity and spectroscopic studies of their stereoisomers and conformers ‡

The compounds NH(SiHMeCl)2 1 and NMe(SiHMeCl)2 2 have been prepared by treating SiHMeCl2 with CaCl2·8NH3 and NH2Me respectively. Each was characterised by elemental analysis, mass spectrometry, NMR and IR spectroscopy. Dipole moments were also measured. The NMR spectra indicate that both compounds form 1∶1 mixtures of the rac and meso diastereomers, their abundances corresponding to a statistically controlled synthetic pathway. The NMR and mass spectra also show that 1,5-dichloro-1,2,3,4,5-pentamethyltrisilazane, SiHMe[NMe(SiHMeCl)]2 3, which is formed as a side product in the synthesis of 2, also consists of two diastereomers. Variable-temperature 1H NMR spectra of NH(SiHMeCl)2 indicate participation of H(N) in hydrogen bonding. The compound is decomposed by heat and reacts with pyridine to form NH4Cl, SiHMeCl2 and polysilazanes, whereas NMe(SiHMeCl)2 shows only slight decomposition up to 80 °C and does not react with pyridine. Infrared spectra in the ν(SiH) region are interpreted in terms of the results of ab initio calculations of frequency, intensity and conformer abundance. The two bands near 2200 cm–1 in the spectrum of 2 have their origin in two effects: different orientations of the Si–H bonds relative to the Si–N–Si plane in several conformers and an unprecedented strong dipole–dipole coupling between the two Si–H bond stretching motions in situations where the bonds are roughly parallel. The absence of such an observed splitting for 1 is likely to be due in part to signal averaging during a free internal rotation. Significant couplings are also calculated to occur between Si–Cl bond-stretching motions, whose source must be different from that for the Si–H bond stretches.

Microwave spectroscopic and ab initio studies of the hydrogen-bonded trimethylamine–hydrogen sulfide complex

Rotational spectra have been recorded for six isotopomers of the trimethylamine–hydrogen sulfide complex using a Fourier-transform microwave spectrometer. The spectra are characteristic of a symmetric top (B+C)/2=1395.463 (1) MHz, and are indicative of free internal rotation of H2S about the trimethylamine symmetry axis. A structure with a single, linear hydrogen bond, with an S–N distance of 3.36 (5) Å, best reproduces the moments of inertia of six isotopomers, including three distinct deuterated complexes. The experimental structure is compared to an ab initio structure optimized at the MP2/6-31+G(d,p) level which predicts an S–N distance of 3.328 Å. MP2/aug′-cc-pVTZ calculations were used to determine the binding energy of the complex (ΔEe=−5.8 kcal/mol) and the barrier to an internal tunneling motion which exchanges the two H2S hydrogens (3.0 kcal/mol).

High resolution Fourier transform infrared spectrum and vibration-rotation analysis of the B-type 1584 cm − band of nitromethane

The high resolution infrared spectrum of nitromethane with a nearly free methyl internal rotor (V 6 = 2.11 cm −1) has been recorded for the B-type band near 1584 cm −1 with a Fourier transform spectrometer having a resolution of 0.005 cm −1. The ro-vibrational analysis of this dominantly NO 2 asymmetric stretching band has led to the identification of 989 transitions with 0 ≤ J ≤ 42 and 0 ≤ K α ≤ 28 for the methyl rotor m = 0 state. The transitions are fitted using Watson's A-reduction Hamiltonian in the I r representation and the least-squares fit of the assigned transitions yield the centre wavenumber and the effective rotation and centrifugal distortion constants of the upper vibrational state ( v = 1).

Molecular structure and intramolecular motion of hexamethyldisiloxane from gas-phase electron diffraction

The molecular structure of hexamethyldisiloxane has been reinvestigated by electron diffraction. The two kinds of large amplitude motion in the molecule, the Si–O–Si bending and the torsion about the Si–O bonds, were treated independently in terms of their potential functions. In the equilibrium structure the molecule has probably C 2v symmetry with staggered C 3Si–O–Si fragments and a nonlinear Si–O–Si linkage with an angle of 155°. The barrier to linearization was obtained to be at least 10 kJ mol −1. In addition, there is free or nearly free internal rotation of the trimethylsilyl groups in hexamethyldisiloxane. In terms of r g bond lengths the electron diffraction analysis yielded Si–O 1.639±0.003 Å, Si–C 1.871±0.004 Å, and the r a angle Si–O–Si 152±2°.

High resolution mid-infrared spectroscopy of ArH2O: The v2 bend region of H2O

Six internal rotation/vibration bands of ArH2O are observed in a slit supersonic expansion via direct absorption of a tunable diode laser in the v2 bend region of H2O. The spectra obtained for the ortho H2O manifold are well represented by a pseudodiatomic model with nearly free internal rotation of the H2O subunit. By way of contrast, the para bands show significant mixing between the internal rotor and stretch states, indicative of strong angular-radial coupling in the intermolecular potential. The spectra for the para Ar–H2O species can be deperturbed based on a three state Coriolis plus angular-radial coupling model which includes microwave, far-ir and near-ir data. The results indicate a redshift of ≈0.58 cm−1 upon bend excitation of the H2O subunit, and in general rather modest changes in the excited state intermolecular potential from the ground state potential. No indication of predissociation broadening is found, and the instrument-limited linewidths place a lower limit on the vibrational lifetime in the excited state of τ⩾7.2(6) ns.

Microwave spectrum and geometry of the methyl isocyanide–argon complex

The rotational spectrum of a complex formed between methyl isocyanide and argon has been observed using pulsed-nozzle, Fourier-transform microwave spectroscopy. The spectrum and geometry of the complex are very similar to those found for CH 3 CN···Ar (R. S. Ford et al., J. Chem. Phys., 1991, 94, 5306). The complex is T-shaped, with the argon atom located 3.64 Afrom the centre of mass of the methyl isocyanide molecule. There appears to be almost free internal rotation of the methyl isocyanide molecule about its symmetry axis in the complex.