The infrared and Raman spectra of 2-cyclohexen-1-ol have been recorded and analyzed. The experimental work has been complemented by ab initio and density functional theory computations. The calculations show that in the vapor phase the conformations with the π-type hydrogen bonding are the lowest in energy, and these findings are supported by the experimental spectra, which agree well with the theoretical predictions. The six conformers predicted result from differences between the direction on the ring-twisting angle and the −OH internal rotation angle. The lowest-energy conformer has the hydrogen of the OH group pointing to the middle of the C═C double bond. The other conformers are calculated to be 72 cm–1 (0.21 kcal/mol) to 401 cm–1 (1.15 kcal/mol) higher in energy. In the liquid phase, only two conformers can be identified in the spectra, and these correspond to different directions of the ring-twisting.

The internal rotation about the single bond connecting a cyclopropyl ring to a CH3, SiH3, GeH3, NH2, SH, or OH group was investigated. Both CCSD/cc-pVTZ and MP2/cc-pVTZ ab initio calculations were performed to predict the structures of these molecules and their internal rotation potential energy functions in terms of angles of rotation. The barriers to internal rotation for the CH3, SiH3, and GeH3 molecules from the calculations agree well with the experimental ones, within −11% to +1% for CCSD/cc-pVTZ and −4% to +9% for MP2/cc-pVTZ. Comparisons between theory and experiment were also performed for propylene oxide and propylene sulfide, and the agreements were very good. Theoretical calculations were performed to compute the internal rotation potential energy function for cyclopropanol, and these were used to guide the determination of a potential function based on experimental data. This molecule has two equivalent synclinal (gauche) conformers with an estimated barrier of 759 cm–1 (9.1 kJ/mol) between them. The minima are at internal rotation angles of the OH group of 109° and 251°. The theoretical potential functions for cyclopropanethiol and cyclopropylamine were also calculated, and these agree reasonably well with previous experimental studies.

The vapor-phase Raman spectra of an atmosphere of cyclohexane vapor heated to 90 and 110 °C collected over a large period of time and utilizing a high laser power of 4 W show hot band series starting at 380.8 cm–1 and corresponding to the v6(A1g) ring-inversion vibration. Fitting this data with a one-dimensional potential energy function allows the barrier to planarity of 8600 cm–1 (24.6 kcal/mol) to be calculated. Ab initio calculations (MP2/cc-pVTZ) predict a value of 10 377 cm–1 (29.7 kcal/mol), while DFT (B3LYP/cc-pVTZ) calculations predict 8804 cm–1 (25.2 kcal/mol).

•The structures of 2,3,5,6-tetrafluoropyridine for its S0 and S1(π, π∗) states have been calculated.•TFPy is rigidly planar in its ground electronic state, but is quasi-planar and floppy in S1.•The barrier to planarity is 30 cm−1 in the excited state.•The observed vibrational frequencies for both states agree well with the computations.•A ring-bending potential energy function for the S1(π, π∗) state was proposed.Infrared and Raman spectra of 2,3,5,6-tetrafluoropyridine (TFPy) were recorded and vibrational frequencies were assigned for its S0 electronic ground states. Ab initio and density functional theory (DFT) calculations were used to complement the experimental work. The lowest electronic excited state of this molecule was investigated with ultraviolet absorption spectroscopy and theoretical CASSCF calculations. The band origin was found to be at 35,704.6 cm−1 in the ultraviolet absorption spectrum. A slightly puckered structure with a barrier to planarity of 30 cm−1 was predicted by CASSCF calculations for the S1(π, π∗) state. Lower frequencies for the out-of-plane ring bending vibrations for the electronic excited state result from the weaker π bonding within the pyridine ring.Graphical abstract

•The infrared and Raman spectra of bicyclobutane vapor and liquid have been recorded and assigned.•The computed spectra from DFT calculations match the experimental spectra very well.•Ab initio calculations were used to obtain the structure of the molecule and this was compared to related bicyclic molecules.•Very distinct infrared band types were observed.•The low-frequency skeletal modes are a reflection of the ring rigidity.The infrared and Raman spectra of vapor-phase and liquid-phase benzocyclobutane (BCB) have been recorded and assigned. The structure of the molecule was calculated using the MP2/cc-pVTZ basis set and the vibrational frequencies and spectral intensities were calculated using the B3LYP/cc-pVTZ level of theory. The agreement between experimental and calculated spectra is excellent. In order to allow comparisons with related molecules, ab initio and DFT calculations were also carried out for indan (IND), tetralin (TET), 1,4-benzodioxan (14BZD), 1,3-benzodioxan (13BZD) and 1,4-dihydronaphthalene (14DHN). The ring-puckering, ring-twisting, and ring-flapping vibrations were of particular interest as these reflect the rigidity of the bicyclic ring system. The infrared spectra of BCB show very nice examples of vapor-phase band types and combination bands.Graphical abstract

The infrared and Raman spectra of the bicyclic spiro molecule 2-cyclopenten-1-one ethylene ketal (CEK) have been recorded. Density functional theory (DFT) calculations were used to compute the theoretical spectra, and these agree well with the experimental spectra. The structures and conformational energies for the two pairs of conformational minima, which can be defined in terms of ring-bending (x) and ring-twisting (τ) vibrational coordinates, have also been calculated. Utilizing the results from ab initio MP2/cc-PVTZ computations, a two-dimensional potential energy surface (PES) was calculated. The energy levels and wave functions for this PES were then calculated, and the characteristics of these were analyzed. At lower energies, all of the quantum states are doubly degenerate and correspond to either the lower-energy conformation L or to conformation H, which is 154 cm–1 higher in energy. At energies above the barrier to interconversion of 264 cm–1, the wave functions show that the quantum levels have significant probabilities for both conformations.

The vapor-phase Raman spectra of cis- and trans-stilbene have been collected at high temperatures and assigned. The low-frequency skeletal modes were of special interest. The molecular structures and vibrational frequencies of both molecules have also been obtained using MP2/cc-pVTZ and B3LYP/cc-pVTZ calculations, respectively. The two-dimensional potential map for the internal rotations around the two Cphenyl–C(═C) bonds of cis-stilbene was generated by using a series of B3LYP/cc-pVTZ calculations. It was confirmed that the molecule has only one conformer with C2 symmetry. The energy level calculation with a two-dimensional Hamiltonian was carried out, and the probability distribution for each level was obtained. The calculation revealed that the “gearing” internal rotation in which the two phenyl rings rotate with opposite directions has a vibrational frequency of 26 cm–1, whereas that of the “antigearing” internal rotation in which the phenyl rings rotate with the same direction is about 52 cm–1. In the low vibrational energy region the probability distribution for the gearing internal rotation is similar to that of a one-dimensional harmonic oscillator, and in the higher region the motion behaves like that of a free rotor.

The infrared and Raman spectra of 2,6-difluoropyridine (26DFPy) along with ab initio and DFT computations have been used to assign the vibrations of the molecule in its S0 electronic ground state and to calculate its structure. The ultraviolet absorption spectrum showed the electronic transition to the S1(π,π*) state to be at 37 820.2 cm–1. With the aid of ab initio computations the vibrational frequencies for this excited state were also determined. TD-B3LYP and CASSCF computations for the excited states were carried out to calculate the structures for the S1(π,π*) and S2(n,π*) excited states. The CASSCF results predict that the S1(π,π*) state is planar and that the S2(n,π*) state has a barrier to planarity of 256 cm–1. The TD-B3LYP computations predict a barrier of 124 cm–1 for the S1(π,π*) state, but the experimental results support the planar structure. Hypothetical models for the ring-puckering potential energy function were calculated for both electronic excited states to show the predicted quantum states. The changes in the vibrational frequencies in the two excited states reflect the weaker π bonding within the pyridine ring.

The infrared and Raman spectra of vapor, liquid, and solid state cyclopentane and its d1, 1,1-d2, 1,1,2,2,3,3-d6, and d10 isotopomers have been recorded and analyzed. The experimental work was complemented by ab initio and density functional theory (DFT) calculations. The computations confirm that the two conformational forms of cyclopentane are the twist (C2) and bent (Cs) structures and that they differ very little in energy, less than about 10 cm–1 (0.1 kJ/mol). The bending angle for the Cs form is 41.5° and the dihedral angle of twisting is 43.2° for the C2 form. A reliable and complete vibrational assignment for each of the isotopomers has been achieved for the first time, and these agree very well with the DFT (B3LYP/cc-pVTZ) computations. The ab initio CCSD/cc-pVTZ calculations predict a barrier to planarity of 1887 cm–1, which is in excellent agreement with the experimental value of 1808 cm–1.