Free Internal Rotation Research Articles

Microwave spectroscopic characterization of a strong hydrogen bond: trimethylamine-water

Rotational spectra were observed for 10 isotopomers of the trimethylamine-water complex with use of a Fourier-transform microwave spectrometer. The observed spectra of all 10 isotopomers were characteristic of a symmetric top and are indicative of a free internal rotation of the water about the symmetry axis of trimethylamine. Analysis of the rotational constants reveals a structure with an essentially linear single hydrogen bond (1.82 A). The dipole moment of the complex was determined to be 2.37 (1) D with use of Stark effect measurements. Ab initio calculations were used to model the structure and intermolecular potential energy surface of the complex

Infrared studies of SiH bonds and the structures of methylsilylamines and ethers

Gas phase IR spectra have been investigated in a series of methylsilyl compounds, including MeSiHDX (X = F, Cl), MeSiHX2 (X = F, Cl), Me2SiHCl, (Me2SiH)2NH, (MeSiH2)3N, (MeSiHD)3N, (MeSiH2)2NMe,* (Me2SiH)2NMe,* MeSiH2NMe2,* Me2SiHNMe2, (MeSiHD)2O and (Me2SiH)2O. In the asterisked molecules, pairs of νSiH bands are seen which confirm the existence of fixed conformations. Amongst the other compounds, the single band seen is attributed on the basis of the α-Me substitutent effect to a fixed conformation in the cases of Me2SiHNMe2 and (Me2SiH)2O. In (Me2SiH)2NH and (MeSiH2)2O, free internal rotation about the SiN and SiO bonds seems likely. Apart from the possible case of (MeSiH2)3N, the spectra appear to be compatible with the electron diffraction evidence, and for the most part in broad agreement with the structures found. 2νSiH bands are in general harder to see in these amines and ethers than in Me2SiHCl. A likely transition to a (1, 1) local mode state is observed in (MeSiH2)3N. Determination of barriers to internal rotation in these molecules should be a high priority.

Hydrogen bonding in the benzene–ammonia dimer

AMINES have long been characterized as amphoteric (acting as both donor and acceptor) in terms of their hydrogen-bond interactions in the condensed phase. With the possible exception of (NH_3)_2, however, no gas-phase complexes exhibiting hydrogen-bond donation by ammonia, the ‘simplest amine’, have been observed. Here we present high-resolution optical and microwave spectra of the benzene–ammonia dimer in the gas phase, which show that the ammonia molecule resides above the benzene plane and undergoes free or nearly free internal rotation. In the vibrationally averaged structure, the C_3 symmetry axis of NH_3 is tilted by about 58° relative to the benzene C_6 axis, such that the ammonia protons interact with the benzene π-cloud. Our ab initio calcula-tions predict a 'monodentate' minimum-energy structure, with very low barriers to rotation of ammonia. The larger separation of the two molecular components, and the smaller dissociation energy, relative to the benzene–water dimer reflect the weak hydrogen-bond donor capability of ammonia, but the observed geometry greatly resembles the amino–aromatic interaction found naturally in proteins.

Molecular ozone-water complex.ab initio calculation with inclusion of electron correlation

The geometrical and electronic structure of different molecular O3−H2O complexes has been calculated by the ab initio method in the 4–31G (d, p) basis set with inclusion of electron correlation according to the Moller-Plesset fourth-order perturbation theory (MP4). It has been shown that the geometrical structure of the experimentally observed hydrogen-bonded complex is mainly determined by the entropy (rather than energy) effect, and is characterized by an almost free internal rotation of the H2O molecule.

Potential energy surfaces of substituted anilines: Conformational energies, deuteration effects, internal rotation, and torsional motion

Mass resolved excitation spectra (MRES) are presented for a series of substituted anilines including 2- and 3-methylaniline, 2- and 3-ethylaniline, 2-aminobenzyl amine, and 2-aminobenzyl alcohol. The observed spectra show the following phenomena: nearly free internal rotation of the methyl substituent in the S1 state; long vibrational progressions attributed to C–C, C–N, and C–O side chain torsional motion; an inequivalence of the two amino hydrogens for both ring and side chain amino groups as determined from the spectra of deuterated species; and the existence of two conformers for 2-aminobenzyl alcohol. Semiempirical and ab initio calculations are performed on these systems to aid in the analysis of the potential energy surfaces and in the interpretation of the experimental results.

Millimeter-wave spectra and revised spectroscopic parameters of methylsilane

Rotational spectra of methylsilane in the ground and several torsionally excited states were measured in the region from 87 to 350 GHz with Doppler-limited resolution. For some of the transitions in the ground vibrational state, the internal rotation splitting has been recorded with sub-Doppler resolution via the saturation technique. Transitions in the torsional states with v t ≤ 2 have been assigned and analyzed with a least-squares procedure, employing a direct diagonalization method for calculating the torsion-rotation energy levels. A phenomenological Hamiltonian has been developed to describe on the free-internal rotor basis the internal and overall rotation of symmetric-top molecules. The potential barrier of methylsilane to internal rotation was determined to be V 3 ≈ 590.5 cm −1.

Intermolecular HF motion in ArnHF micromatrices (n=1,2,3,4): Classical and quantum calculations on a pairwise additive potential surface

The availability of pairwise additive ‘‘two-body’’ potentials for van der Waals systems from near-IR, far-IR and microwave data permits detailed prediction of librational behavior for isolated HF chromophores solvated by successive numbers of rare gas Ar atoms. This paper describes theoretical calculations of ArnHF equilibrium structures and intermolecular HF vibrational frequencies based on an ArnHF ‘‘two-body’’ potential energy surface developed from previously determined Ar–Ar and Ar–HF potentials. Isomeric structures are predicted from local minima on these multidimensional surfaces, and are found to be in excellent qualitative agreement with near-IR observations of ArnHF clusters with n=1,2,3, and 4 Ar atoms. Quantum mechanical calculations are performed for the HF librational and van der Waals stretching modes against a rigid Arn frame. These pairwise additive potentials predict a strongly increasing angular anisotropy for the HF bending coordinate with number of Ar atoms (for small n), and provide predictions of HF intermolecular van der Waals bend and stretch vibrational frequencies. Fourier transform (FT)-microwave and near-IR data, on the other hand, demonstrate only a minor dependence of the anisotropy on n; this suggests the pairwise additive potentials may systematically overestimate the angular anisotropy for HF bending. Selected cuts through these potential surfaces indicate significant coupling between the Arn–HF stretch, Ar–Ar stretch, and Ar–Ar bending vibrations. This strong vibrational coupling indicates that a full quantum treatment of all intermolecular coordinates may be required in order to make quantitative comparison with van der Waals vibrational data. In the limit of sufficient Ar atoms to fill the first coordination sphere around the HF, the calculations indicate a nearly perfect cancellation of angular anisotropy for HF librational motion, consistent with the nearly free internal rotation of the HF observed in cryogenic Ar matrices.

Microwave spectrum of benzene⋅SO2: Barrier to internal rotation, structure, and dipole moment

The microwave spectrum of the benzene⋅SO2 complex was observed with a pulsed beam Fourier-transform microwave spectrometer. The spectrum was characteristic of an asymmetric-top with a- and c-dipole selection rules. In addition to the rigid-rotor spectrum, many other transitions were observed. The existence of a rich spectrum arose from torsional–rotation interactions from nearly free internal rotation of benzene about its C6 axis. Transitions from torsional states up to m=±5 were observed. The principal-axis method (PAM) internal rotation Hamiltonian with centrifugal distortion was used to assign the spectrum. Assuming six-fold symmetry for the internal rotation potential, the barrier height was determined as V6=0.277(2) cm−1. The spectrum of C6D6⋅SO2 was also assigned. Analysis of the moments of inertia indicated that the complex has a stacked structure. The distance Rcm separating the centers of mass of benzene and SO2, as well as the tilt angles of the benzene and SO2 planes relative to Rcm were determined. The values obtained were Rcm=3.485(1) Å, θC6H6=±12(1)° and θSO2=44(6)°. While SO2 is certainly tilted with the sulfur end towards benzene, the sign of the benzene tilt angle could not be unambiguously determined. The dipole moment of C6H6⋅SO2 was determined as μa=1.691(2) D, μc=1.179(2) D, and μT=2.061(2) D.

Benzene Forms Hydrogen Bonds with Water

Fully rotationally resolved spectra of three isotopic species of 1:1 clusters of benzene with water (H(2)O, D(2)O, and HDO) were fit to yield moments of inertia that demonstrate unambiguously that water is positioned above the benzene plane in nearly free internal rotation with both hydrogen atoms pointing toward the pi cloud. Ab initio calculations (MP2 level of electron correlation and 6-31 G(**) basis set with basis set superposition error corrections) predict a binding energy D(e) greater, similar 1.78 kilocalories per mole. In both the experimental and theoretical structures, water is situated nearly 1 angstrom within the van der Waals contacts of the monomers, a clear manifestation of hydrogen bond formation in this simple model of aqueous-pi electron interactions.

Vibrational spectra and conformations of 1, 4‐dibromobut‐2‐yne and 1,4‐diiodobut‐2‐yne

AbstractThe Raman spectra of 1,4‐dibromobut‐2‐yne (BrBr) as a liquid at various temperatures and as an amorphous and crystalline solid at low temperatures were recorded in the 3200‐10 cm−1 region. IR spectra of the liquid at room temperature and of the amorphous and crystalline solids at low temperatures (4000‐50 cm−1) were obtained. Additional IR spectra of the vapour were recorded (4000‐500 cm−1). The spectra of the liquid phase showed characteristic, broad bands below 500 cm−1. No temperature dependence was detected in these, and the spectra of the amorphous solid at 90 K are almost identical with the room temperature spectra of the liquid. A shoulder on the Rayleigh line was observed at ca. 20 cm−1 in the Raman spectrum of the amorphous BrBr at 90 K which tentatively, has been interpreted as the torsional mode. In the crystal an anti conformer was present, while the spectra of the vapour and the liquid were interpreted in terms of large torsional freedom. The Raman and IR spectra of the very unstable 1,4‐diiodobut‐2‐yne (II) as a crystalline solid were recorded in the 4000‐20 cm−1 range at low temperatures. Spectra of carbon disulphide solutions at temperatures 250–260 K were obtained and additional IR spectra of II in Nujol mulls and in KBr pellets were also recorded. In the region below 500 cm−1, Where other 1,4‐dihalobut‐2‐ynes have characteristic, broad bands, a high irregular background was observed in the Raman spectrum of II in solution. The spectra agree with an anti conformer present in the crystal and nearly free internal rotation in the liquid. The vibrational assignments are supported by force constant calculations and by a correlation diagram for the vibrations of the six skeletal bending modes in 1,4‐dichlorobut‐2‐yne, BrBr and II.

Vibrational spectra and conformation of 1‐chloro‐4‐fluorobut‐2‐yne and ab initio calculations on 1,4‐dihalobut‐2‐ynes

AbstractRaman spectra were recorded in the 3200‐5 cm−1 range for 1‐chloro‐4‐fluorobut‐2‐yne as a liquid at various temperatures and as amorphous and different crystalline solids at low temperature. In the liquid, the Raman spectra showed broad, asymmetrical band shapes as previously reported for other 1,4‐dihalobut‐2‐ynes. Additional IR and far‐IR spectra were obtained of the liquid, vapour, amorphous and crystalline solids at low temperature, and of the compound isolated in argon and nitrogen matrices at 13 K. The vapour and liquid spectra were interpreted in terms of a low barrier to internal rotation and the crystal spectra of a single conformer, gauche. The unusual band shapes in the low‐frequency Raman spectra of the 1,4‐dihalobut‐2‐ynes are discussed and are reproduced fairly well by a simple model of a molecule exhibiting nearly free internal rotation. The agreement between the observed and calculated spectra is good for all the molecules studied when ab initio potentials are applied in the model calculations. Temperature effects in the Raman spectra of liquid 1‐chloro‐4‐fluorobut‐2‐yne can also be derived from the model. Internal rotation in 1,4‐difluoro‐, 1,4‐dichloro‐, 1‐chloro‐4‐fluoro‐, 1,4‐dibromo‐ and 1,4‐diiodobut‐2‐yne was studied with the aid of ab initio calculations. The barriers were found to be low and decreasing through the series from 1,4‐difluoro‐ to 1,4‐diiodobut‐2‐yne accompanied by an increase in the equilibrium dihedral angle, XCCCCY, from 101° in 1,4‐difluorobut‐2‐yne to 180° in 1,4‐diiodobut‐2‐yne. The systematic variations through the series were correlated with the local dipole moments of the halomethyl groups and the electronegativities of the halogen substituents.

The spectroscopic and photophysical effects of the position of methyl substitution. I. 4- and 5-methylpyrimidine

Laser-induced fluorescence excitation and dispersed fluorescence spectra of the first n–π* transition of jet-cooled 4- and 5-methylpyrimidine (4-mp and 5-mp) have been recorded and analyzed. In 5-mp, methyl substitution preserves many of the spectroscopic signatures of the unsubstituted pyrimidine molecule. Dispersed fluorescence spectra are used to assign most of the major features in the first 1000 cm−1 of the excitation spectrum. High resolution scans at the origin reveal a 0.20 cm−1 splitting of the origin arising from the 0a′1–0a1 and 1e″–1e″ internal rotor transitions. From this small splitting we deduce that the nearly free internal rotation of the methyl group in the ground state is carried over to the S1 state as well. In 4-methylpyrimidine, the reduction in symmetry accompanying methyl substitution (G12 to G6 ) results in allowed transitions to all in-plane fundamentals. The methyl group is seen to participate in the electronic transition to a greater degree in 4-mp than in 5-mp. We observe clear activity in both the C–CH3 stretch and C–CH3 in-plane bend in the dispersed fluorescence from the origin of 4-mp. 4-mp also differs remarkably from 5-mp in the magnitude of the barrier to methyl internal rotation in S0 and S1. By fitting the positions and intensities of internal rotor structure in ground and excited states we deduce a ground state barrier to internal rotation of V″3=95±5 cm−1 and a best-fit excited state barrier of V3=745 cm−1, V′6=−100 cm−1. Ab initio calculations on 4-mp which reproduce both the magnitude and shape of the experimental barrier to internal rotation in the ground state. The lowest energy methyl conformation places a hydrogen atom in the plane of the ring pointing away from the nitrogen lone pair. Finally, in both molecules we observe spectroscopic signatures of vibrational state mixing in the S1 state. Density-of-states calculations on both molecules using the experimentally determined internal rotor energy levels predict a similar density of same-symmetry states in the two molecules at a given energy. Experimental evidence is presented that the small V6 barrier in 5-mp leads to modest vibration/internal rotation coupling matrix elements of ∼1 cm−1. The high V3 barrier in 4-mp is observed to give strong vibration/internal rotation coupling in the case of the 6a10(0a1) level which shifts its position by 17 cm−1 from its companion 6a10(1e) level due to interaction with an X1(3a2) vibration/internal rotation combination band.

Vibrational perturbation in the ground and first vibrationally excited states of trans methyl nitrite

In an N-atomic molecule with a nearly free internal rotation the interaction of the 3N-7 vibrational modes with the rotation-internal rotation motion produces a modification of the rotation and internal rotation constants in the Hamiltonian. The modifications of these constants were explicitly deduced by us (1) in terms of the Coriolis coupling between vibration and overall rotation and internal rotation and torsional angle derivatives of the normal coordinates. In this work we have numerically evaluated the modification of the rotation-internal rotation constants in the ground and first vibrationally excited states (v 14 =1) of trans methyl nitrite which has a very low barrier (V 3 =10 cm −1 ) methyl rotor (2)

The dynamics of open-shell Van der Waals complexes

The theory of Van der Waals complexes formed from atoms and open-shell (Σ and Π) diatomic molecules is developed, paying particular attention to the quantum numbers that are conserved in the complex and the angular momentum coupling cases that may be observed. Complexes formed from diatoms in multiplet Σ states may exhibit several different coupling schemes closely analogous to Hund’s coupling cases for diatomic molecules. Complexes formed from diatoms in Π states usually exhibit a coupling scheme in which the (signed) projection P of the diatom angular momentum j onto the intermolecular axis is nearly conserved. Correlation diagrams showing the bending energy levels as a function of potential anisotropy are given for complexes containing diatomic molecules in both Σ and Π states. The transition from free internal rotor quantum numbers to near-rigid bender quantum numbers with increasing anisotropy is investigated. The cases of Ar–OH and Ne–OH are considered as examples.

HF-ClF(g) molecular complex: two limiting approaches to its thermodynamics

An evaluation has been carried out of the thermodynamics of the gas-phase association of HF and C1F which gives two isomeric complexes, HF · ClF and ClF · HF. The evaluation has been carried out in terms of the RRHO partition functions and the energetics supplied with molecular data from recent quantum-chemical calculations. Special attention has been paid to the internal rotational motion in these complexes which, as well as being described as a harmonical torsion, has also been treated by another limiting approach, namely the free internal rotation. To describe the thermodynamics of the latter motion, the technique of direct summation, recently introduced for these purposes, was used. Under certain conditions both limiting approaches to this motion can lead to relatively great differences in the values of the thermodynamic terms. Special attention has been paid to the interplay of both isomeric structures within their equilibrium mixture which is characterized in detail. In particular it has been shown that the order of the relative stabilities of both isomers can be interchanged with temperature.

Is the carbon monoxide—acetylene dimer semirigid? Evidence from its rotational spectrum

The ground-state rotational spectra of ten isotopomers of the carbon monoxide—acetylene dimer are reported and the spectroscopic constants B O and D J determined in the usual way in a semirigid rotor fit for a linear molecule. The B O values are consistent with a simple linear model of the dimer with nuclei in the order OC…HCCH. The failure to observe the isotopomer OC…HCCD in mixtures of CO and HCCD is interpreted in terms of a zero-point energy difference rather than in terms of a nearly free internal rotation of the acetylene subunit.

Rotational spectrum and structure of the complex Ar–CH3CN

The microwave spectrum of the weakly bound complex Ar–CH3CN has been observed using a pulsed-nozzle Fourier-transform microwave spectrometer. The spectrum is characteristic of an asymmetric rotor with nearly free internal rotation of the methyl group. Spectroscopic constants for the ground internal rotor state, in megaHertz, are 3:[RW3:A=9323.7769(22),:B+C=3439.5578(15),:B–C=326.6860(12)].

The submillimeter rotation-tunneling spectrum of Ar-D 2O and Ar-NH 3

Two bands for Ar-D 2O and one band for Ar-NH 3 have been observed for the first time in the frequency region 300–500 GHz. For both complexes the bands are explained in terms of a nearly free internal rotor model, following earlier investigations of these types of complexes. We found evidence that this model is correct for Ar-D 2O. Two of the internal rotor states studied for the Ar-D 2Ocomplex show a Coriolis interaction and the interaction constant is determined. The hyperfine constants for the studied excited state of the Ar-NH 3 complex turn out to be large.

Preliminary structural characterization of complexes of cyanogen: NH3-NCCN and (NCCN)2

The complexes of NCCN with itself and with NH 3 have been studied by molecular beam electric resonance spectroscopy. In both species a heavy-atom arrangement of C 2v symmetry is observed. The NH 3 -NCCN complex appears to exhibit essentially free internal rotation of the NH 3 subunit about its internal symmetry axis

Vibrational spectroscopy of the ammoniated ammonium ions NH4+(NH3)n (n = 1-10)

The gas-phase vibration-internal rotation spectra of mass-selected ammoniated ammonium ions, NH{sub 4}{sup +}(NH{sub 3}){sub n} (for n = 1-10), have been observed from 2600 to 4000 cm{sup {minus}1}. The spectra show vibrational features that have been assigned to modes involving both the ion core species, NH{sub 4}{sup +}, and the first shell NH{sub 3} solvent molecules. Nearly free internal rotation of the solvent molecules about their local C{sub 3} axes in the first solvation shell has been observed in the smaller clusters (n = 1-6). For the larger clusters studied (n = 7-10) the spectra converge, with little difference between clusters differing by one solvent molecule. For these clusters, the spectrum in the 3200-3500 cm{sup {minus}1} region is quite similar to that of liquid ammonia, and the entire region of 2600-3500 cm{sup {minus}1} also bears considerable resemblance to the spectra of ammonium salts dissolved in liquid ammonia under some chemical conditions. This indicates the onset of a liquidlike environment for the ion core and first shell solvent molecules in clusters as small as NH{sub 4}{sup +}(NH{sub 3}){sub 8}.