Ring-puckering Potential Function Research Articles

The equivalence of the methyl groups in puckered 3,3-dimethyloxetane

3,3-dimethyloxetane (DMO) has a nonplanar ring equilibrium configuration (ϕ=17.5°) and a double minimum potential function, with a barrier of 47cm−1, for ring-puckering vibration. In a previous work, the observation of endocyclic 13C and 18O monosubstituted isotopologues allowed to conclude that the ring is puckered, consistent with the observed ring-puckering potential function. However, an interesting feature was observed for the 13C substitutions at the methyl carbon atoms. While two different, axial and equatorial, 13C-methyl groups spectra are predicted from a rigid nonplanar ring DMO model, only one spectrum was found. This behavior evidences that the two methyl groups of DMO are equivalent as could be expected for a planar ring. In this work we show how consideration of the potential function and the dynamical path for ring-puckering motion allows to reproduce the experimental results.

Microwave Spectra, Structure, and Ring-Puckering Vibration of Octafluorocyclopentene.

The rotational spectra of octafluorocyclopentene (C5F8) has been measured for the first time using pulsed jet Fourier transform microwave spectroscopy in a frequency range of 6 to 16 GHz. As in the molecule cyclopentene, the carbon ring is nonplanar, and inversion through the plane results in an inversion pair of ground state vibrational energy levels with an inversion splitting of 18.4 MHz. This large amplitude motion leads to the vibration-rotation coupling of energy levels. The symmetric double minimum ring-puckering potential function was calculated, resulting in a barrier of 222 cm-1. The rotational constants A0 = 962.9590(1) MHz, B0 = 885.1643(4) MHz, C0 = 616.9523(4) MHz, A1 = 962.9590(1) MHz, B1 = 885.1643(4) MHz, C1 = 616.9528(4) MHz, and two centrifugal distortion constants for each state were determined for the parent species and all 13C isotopologues. A mixed coordinate molecular structure was determined from a least-squares fit of the ground state rotational constants of the parent and each 13C isotopologue combined with the equilibrium bond lengths and angles from quantum chemical calculations.

Ring planarity problem of 2-oxazoline revisited using microwave spectroscopy and quantum chemical calculations.

In a previous infrared, Raman, and microwave spectroscopic work,1 it was claimed that 2-oxazoline has a planar ring equilibrium conformation, and the ring-puckering potential function V(z) = 22.2(z(4) + 1.31z(2)) cm(-1), where z is a dimensionless reduced coordinate, was derived. This function poorly reproduces the rotational constants of the lowest and most important puckering states. The microwave spectrum has been reinvestigated and largely extended to include more than 4600 transitions of the ground state and six excited states of the ring-puckering vibration allowing accurate centrifugal distortion constants to be obtained for the first time. A new potential function V(z) = 38.8(z(4) - 0.65z(2)) cm(-1) has been determined. This function yields much better agreement between calculated and observed rotational constants, especially for the lowest puckering states, than the previous function and predicts a nonplanar ring equilibrium conformation. The barrier to ring planarity is determined to be 49(8) J/mol. The ground-state energy level is 35 cm(-1) above the barrier maximum. Theory predicts that three of the five Watson centrifugal distortion constants, ΔJK, ΔK, and δK, should vary with the puckering state, whereas ΔJ and δJ should be unaffected. It was found that ΔJK and ΔK indeed behave in the expected manner, while deviations were seen for the three other centrifugal distortion constants. The ab initio methods HF, MP2, CCSD, CCSD(T), and CCSD(T)-F12 with large basis sets as well as several DFT methods were used in an attempt to reproduce the low experimental barrier to the planar ring. Only the MP2 method yielded a satisfactory prediction of the barrier. The CCSD and the CCSD(T) calculations predict a planar ring, whereas the energy differences between a planar and a nonplanar ring obtained in the CCSD(T)-F12 computations are so small that a definite conclusion cannot be drawn.

Theoretical calculations, far-infrared spectra and the potential energy surfaces of four cyclic silanes

Theoretical computations have been carried out to calculate the potential energy functions for the out-of-plane vibrations of four cyclic silanes, and the results were compared to experimental functions determined from far-infrared data. The experimental and computed ring-puckering potential functions for 1-silacyclopent-3-ene, which are in excellent agreement, are quartic in nature with tiny barriers to planarity. Similarly, the calculated and experimental potential functions for 1,3-disilacyclopent-3-ene are nearly identical. For silacyclopentane and 1,3-disilacyclopentane the calculations predict ring-twisitng barriers of 2493cm−1 (vs. 2110cm−1 observed) and 1395cm−1, respectively. The conformational energies for the bent forms were calculated to be 1467cm−1 (vs. 1509cm−1 observed) for the former and 878cm−1 for the latter relative to the energy of the twist minima. One-dimensional hindered pseudorotational potential energy functions were found to work well for predicting the observed far-infrared spectra for the bending (pseudorotational) vibration.

Laser induced fluorescence and ultraviolet absorption spectra and the ring-puckering potential function of 1,4-dihydronaphthalene in its ground and S 1(π, π ∗) electronic states

The fluorescence excitation spectra, single vibrational level fluorescence spectra, and ultraviolet absorption spectra of 1,4-dihydronaphthalene have been analyzed. The first four quantum energy spacings for the ring-puckering vibration were determined for the S 0 and S 1(π, π ∗) electronic states, and the potential energy functions were also determined. In the ground state the single-minimum function is dominated by the quartic term, but in the excited state the function becomes nearly harmonic. The S 0 function is less rigid than that for 1,4-cyclohexadiene but the S 1(π, π ∗) function becomes much more rigid. Earlier work on this molecule had erroneously postulated barriers in both electronic states.

Conformational stability, structural parameters, and vibrational frequency assignments of ethylcyclobutane using infrared and Raman spectroscopy and ab initio calculations

Mid-infrared spectra (3500–400cm−1) of gaseous and polycrystalline solid ethylcyclobutane have been recorded, as well as the far-infrared (400–50cm−1) spectrum of ethylcyclobutane in the annealed solid phase. Additionally, the Raman spectra (4000–20cm−1) of liquid and annealed solid ethylcyclobutane have also been obtained. Spectroscopic evidence for the coexistence of the equatorial–gauche, axial–gauche and equatorial–cis conformers of the title compound is found in the fluid phase, with the equatorial–gauche rotamer the most stable form, and the only rotamer present in the annealed solid. Ab initio calculations have been carried out with different basis sets up to MP2/6-31G∗, from which the structural parameters, conformational stabilities, force constants, infrared and Raman intensities, and vibrational frequencies have been determined. By incorporating the results from the ab initio calculations, a normal coordinate analysis of the title compound has also been performed and this aids in the vibrational assignments of the normal modes. The weak features at 172 and 161cm−1 in the far-IR and Raman spectra of the solid have been assigned to the ring puckering vibrations, and a theoretically predicted ring-puckering potential function has been determined.

Rotational spectrum, ring-puckering vibration and ab initio calculations on 1,1-difluorocyclobutane

The rotational spectra of the ground and first five ring-puckering excited states of 1,1-difluorocyclobutane have been investigated in the centimeter- (8–40 GHz) and millimeter-wave regions (96–106 and 144–159 GHz). Analysis of the spectra for the pairs of coupled vibrational states vp=0/1 and vp=2/3 has been carried out using two-state Hamiltonians, yielding accurate rotational and centrifugal distortion constants, vibration-rotation coupling parameters, and the energy spacings ΔE01 and ΔE23. The spectrum for the vp=4 to vp=6 ring-puckering states was satisfactorily accounted for in terms of effective semi-rigid Hamiltonians for each vibrational state. A double minimum ring-puckering potential function with a barrier to ring inversion of 231(4) cm−1 has been obtained from the analysis of the vibrational dependence of the rotational constants and inversion splittings ΔE01 and ΔE23. The calculated variation of the quartic centrifugal distortion constants with the ring-puckering quantum number reproduces satisfactorily the experimental trends, confirming the validity of the derived potential function. Ab initio calculations with HF, MP2, and B3LYP density functional hybrid methods have been carried out for this molecule using different basis sets. The experimental and ab initio potential functions, coupling terms, and ring-puckering dynamical parameters are compared. Finally, an ab initio near-equilibrium structure is presented.

The Millimeter-Wave Spectrum of 2,3-Dihydrofuran

The millimeter-wave spectrum of 2,3-dihydrofuran in the ground and five ring-puckering excited states has been measured in the frequency range 100–250 GHz. The ground and first ring-puckering excited states have been fitted to a two-state Hamiltonian including Coriolis coupling interaction. The determined energy difference of 18.684(7) cm−1between these states and theaandbtype coupling parameters are consistent with the ring-puckering potential function and the previously observed dependence of the centrifugal distortion constants ΔJK, ΔK, and δK. A small ring-puckering dependence of the quartic centrifugal distortion constants ΔJand δJhas been also observed. This dependence is well accounted for in terms of the ring-puckering potential function and the vibrational dependence of the rotational constants.

The millimetre-wave spectrum of oxetane

The millimetre-wave spectrum of oxetane in the ground state and the first seven vibrational excited states of the ring-puckering motion has been measured in the frequency range 80–250 GHz. A small vibrational dependence of the centrifugal distortion constants Δ J and δ J has been observed. This dependence is well accounted for in terms of the ring-puckering potential function and the vibrational dependence of the rotational constants. Measurements of the electric dipole moment for the ground state and the first four ring-puckering excited states are also reported.

The millimetre-wave spectrum of trimethylene sulphide Vibration-rotation coupling between thev=2 andv=3 ring-puckering excited states

The microwave spectra of trimethylene sulphide in the ground and the first eight excited states of the ring-puckering vibration have been measured in the frequency region 26·5‥–250 GHz. Very large Coriolis coupling interaction effects have been observed for the high-J spectra of the v=2 and 3 ring-puckering states in addition to those previously observed between the states v=0 and v=1. A two-state vibration-rotation interaction analysis of these effects has allowed the determination of a vibrational energy spacing ΔE 23=11·97304(1) cm−1. The ring-puckering potential energy function for this molecule has been determined from a simultaneous weighted least-squares fit of the rotational constants and vibrational energy intervals. The observed ring-puckering dependence of the quartic centrifugal distortion constants, including a small dependence of Δ J and δ J , is well accounted for in terms of the ring-puckering potential function obtained in this work.

The Rotational Spectrum of Methylene Cyclobutane

The rotational spectra of methylene cyclobutane in the ground and first eight excited states of its ring-puckering vibration have been measured in the frequency range 26.5-206 GHz. A two-state analysis of the vibration-rotation interaction for the v = 0 and v = 1 states gives an energy separation between these states of 1.12129(1) cm−1. The ring-puckering vibrational dependence of the rotational constants and the quartie centrifugal distortion constants have been analyzed in terms of a reduced vibrational Hamiltonian with kinetic energy terms including the ring-puckering dependence on the reduced mass. A reduced ring-puckering potential function has been determined with a barrier to the planar configuration of 136 ± 3 cm−1, which is also consistent with the vibrational energy spacings presiously observed.

The conformation and ring-puckering vibration of 2-methyloxetane from microwave spectroscopy and ab initio computations

The conformation and ring-puckering vibration of 2-methoxyloxetane have been studied using microwave spectroscopy and ab initio computations. The microwave spectra of the ground and first four excited states of the ring-puckering vibration have been observed in the frequency range 8–40 GHz and the rotational and quadratic centrifugal distortion constants have been determined. Vibrational energy separations have been obtained from relative intensity measurements. The electric dipole moment (in D) of the ground vibrational state has been determined from Stark effect measurements as μ a = 0.0187 (5), μ b = 1.852 (2), μ c = 0.08 (3), and μ t = 1.854 (3). The vibrational energy separations and the vibrational dependence of the rotational constants suggest that the ring-puckering vibrational has an asymmetric single minimum potential function and also gave a revised assignment of the far infrared spectrum (J. Mol. Struct. 56 (1979) 157). A combined fit to the rotational constants, vibrational energy separations and far infrared vibrational frequencies has been used to determine a reduced potential for the ring puckering vibration. The partial derivatives of the rotational constants with respect to the reduced ring-puckering coordinate show that the equilibrium conformation has the methyl group in the equatorial position. Ab initio computations of the ring-puckering potential function have been made using 6-31G * orbitals and full geometry optimization. These computations also show the molecule to have an asymmetric single minimum potential function with an equatorial equilibrium conformation.

The centimeter and millimeter microwave spectrum of cyclobutanone

The rotational spectra of cyclobutanone in the ground and the first seven excited states of the ring-puckering vibration, ν 27, have been measured in the frequency range 18–250 GHz. The spectra of the ring-puckering excited states with v 27 = 8, 9, and 10 and of 11 vibrational excited states belonging to ring-puckering progressions based on the first excited states of the out-of-plane, ν 26, and in-plane, ν 20, carbonyl bending vibrations have been measured in the frequency range 18–50 GHz. The dependence of the rotational and centrifugal distortion constants on the ring-puckering quantum number has been analyzed in terms of two different reduced ring-puckering potential functions: a mixed quadratic-quartic function and a harmonic oscillator perturbed by a quartic and a Gaussian term. This analysis shows that a one-dimensional model for the ring-puckering is only adequate to describe the features of the low energy levels of this vibration.

A Raman study of several isotopic derivatives of cyclopentene in condensed states

The Raman spectra of various isotopic derivatives of cyclopentane have been studied from 20 to 293 K as functions of temperature and isotropic substitution. In the solid phase I, two paks between 130 and 180 cm-1, assigned to the ring-puckering mode, allow the ring-puckering potential function to be determined in the one-dimensional approximation. Compared with the gas phase, it shows only a slight increase in the barrier height, smaller for monohydrogenated -3h1 cyclopentene (Cy3H) than for the perhydrogenated cyclopentene (CyH8), and a simultaneous decrease in the puckering angle value, more pronounced for Cy3H than the CyH8. These trends could explain the appearance of a new solid phase I', characterized by a planar conformation of the cyclopentene ring, in the multideuterated cyclopentene derivatives. In the solid I and plastic phases, the nu (CH) stretching spectra of monohydrogenated -3h1 (Cy3H) and -4h1 (Cy4H) cyclopentenes show two distinct nu (CH) bands assigned to the CH bond in the axial and equatorial positions. The coalescence of these two bands in the liquid state indicates an exchange rate between the two conformers of about 5*1012 s-1 at 295 K.

The microwave spectrum and ring-puckering vibration of 3-methyloxetane

The microwave spectrum of 3-methyloxetane has been observed in the frequency range 12.4–40.0 GHz in order to investigate the conformational behaviour of this molecule. The spectra of the ground vibrational state, the first five excited states of the ring-puckering vibration and four other excited vibrational states have been assigned and their rotational and quartic centrifugal distortion constants have been obtained. The ring-puckering vibrational energy separations obtained from microwave relative intensity measurements do not appear to be consistent with those predicted from a ring-puckering potential function obtained from far infrared spectroscopy. A revised assignment of the far infrared spectrum gives a potential function consistent with the microwave relative intensity measurements. This potential function is preferred because of its superior ability to account for the ring-puckering vibrational dependence of the rotational and centrifugal distortion constants. Although this is a slightly asymmetric double minimum potential function, the top of the barrier lies below the lowest vibrational level and therefore in its ground vibrational state 3-methyloxetane effectively has a planar ring. The four excited states which do not belong to the ring-puckering progression based on the ground vibrational state are assigned to a ring-puckering progression based on the first excited state of the methyl torsion. The first two members of this progression show relatively large changes in their rotational constants relative to the ground vibrational state indicating a strong interaction between the ring-puckering and methyl torsion vibrations. The electric dipole moment has been determined from Stark effect measurements as μ a = 1.910 (5) D, μ c = 0.77 (2) D and μ = 2.06 (1) D for the ground vibrational state. The μ a component and the total electric dipole moment decrease with excitation of the ring-puckering vibration.

ChemInform Abstract: Vibration‐Rotation Interactions and Ring‐Puckering in 3,3‐Dimethyl Oxetane by Microwave Spectroscopy.

Ring puckering in 3,3-dimethyl oxetane has been investigated using microwave spectroscopy. Microwave spectra of the ground state, the first six ring-puckering excited states, and nine excited states of the methyl groups' deformation vibrations have been observed. The μa electric dipole moment component has been determined as 2.03(3) D from Stark-effect measurements. The vibrational dependence of the rotational constants is consistent with the ring-puckering potential function derived by Duckett et al. (J. Mol. Spectrosc. 69, 159–165 (1978)). Coriolis coupling interactions have been observed and are satisfactorily accounted for with a quartic centrifugal distortion Hamiltonian. The vibrational dependence of the centrifugal distortion constants has been analyzed using the theory developed by Creswell and Mills. In order to reproduce the experimental value of the vibration-rotation interaction parameter, δμabδQ, a dynamical model allowing the rocking of the CH3CCH3 group should be used. The equilibrium ring puckering angle calculated with this model and the ring-puckering potential function is 17.5°.

Vibration-rotation interactions and ring-puckering in 3,3-dimethyl oxetane by microwave spectroscopy

Ring puckering in 3,3-dimethyl oxetane has been investigated using microwave spectroscopy. Microwave spectra of the ground state, the first six ring-puckering excited states, and nine excited states of the methyl groups' deformation vibrations have been observed. The μ a electric dipole moment component has been determined as 2.03(3) D from Stark-effect measurements. The vibrational dependence of the rotational constants is consistent with the ring-puckering potential function derived by Duckett et al. ( J. Mol. Spectrosc. 69, 159–165 (1978)). Coriolis coupling interactions have been observed and are satisfactorily accounted for with a quartic centrifugal distortion Hamiltonian. The vibrational dependence of the centrifugal distortion constants has been analyzed using the theory developed by Creswell and Mills. In order to reproduce the experimental value of the vibration-rotation interaction parameter, δμ ab δQ , a dynamical model allowing the rocking of the CH 3CCH 3 group should be used. The equilibrium ring puckering angle calculated with this model and the ring-puckering potential function is 17.5°.

Theoretical study of the structure and ring inversion in trimethylene sulphone

The structure and ring-puckering potential function of trimethylene sulphone have been obtained within the framework of ab initio SCF calculations. The importance of including polarization functions for the sulphur atom is verified. Computations have shown that trimethylene sulphone is a puckered four-membered ring with a double minimum potential function, which is in agreement with microwave data. Several flexible models for describing the ring-puckering motion have been tested and predicted molecular parameters are compared with experimental values.

Vibration–rotation interactions and ring-puckering potential function in trimethylene sulphone

The microwave spectrum of trimethylene sulphone has been studied in the ground and five vibrational excited states of the ring-puckering vibration. Very large deviations from the rigid rotor behavior were observed for the lowest pair of states. These perturbations resulted from a strong Coriolis coupling between them and have been analyzed using a reduced axis system Hamiltonian. The energy separation between the v=0 and v=1 levels has been determined from this analysis to be of 0.9106±0.0004 cm−1. Small deviations from rigid rotor spectrum were observed for the v=2 to v=5 ring-puckering states that were found to fit semirigid rotor theory where the quartic centrifugal distortion constants account for the vibration–rotation interaction effects. From the variation of the rotational constants and the vibrational separation between v=0 and v=1 states, a reduced ring-puckering potential function has been determined to be V(X)=6.62(X4−9.22X2), which gives a barrier to the planar configuration of 140±35 cm−1. With this potential function the vibration–rotation interaction effects observed for states v=2 to v=5 have been interpreted. Considerations about the ring conformation and the dynamics of the ring-puckering motion have also been made. The μa component of the electric dipole moment has been determined to be 4.8 D.

Microwave investigation and the ring puckering potential function of 3-methylthietan

The microwave spectrum of 3-methylthietan was investigated in the frequency range 26.5–40 GHz. The rotational spectra were measured for the ground state, five puckering states (including the lowest state of the second conformer), and two methyl torsion excited states. The conformer with the methyl group in the equatorial orientation (the ground state, v p = 0) is more stable than the axial counterpart ( v p = 1) by 100(17) cm −1 (1.2(2) kJ/mol). The electric dipole moment was determined for both conformers by measuring the Stark effect on several transitions. A V 3 barrier of 1470(30) cm −1 (17.6(4) kJ/mol) was obtained for the internal rotation of the axial methyl group from the A- E doubling of several rotational transitions in the first torsional excited state. Earlier far-infrared assignments were revised on the basis of microwave intensity measurements. The flexible model approach was used to reproduce the spacings between puckering levels simultaneously with the variation of the rotational constants with puckering excitation, allowing some structural relaxation accompanying the ring deformation. Comparisons are made with the results obtained when an asymmetric one-dimensional puckering Hamiltonian is used.