Ring Bending Mode Research Articles

The microwave spectrum of 2(5 H)-thiophenone

The microwave spectrum of 2(5 H)-thiophenone has been analysed in the frequency range 26.5–40 GHz. R-branch and Q-branch transitions allowed by μ a and μ b have been assigned for the ground state, the first four excited states of the ring-bending vibration, ν B, and the first excited state of the ring-twisting mode, ν T. Relative intensity measurements give the vibrational separations u B = 1 ← 0 and v T = 1 ← 0 as 136 ± 15 and 295 ± 28 cm −1, respectively. The observed value for the quantity Δ 0 = I 0 c − I 0 a − I 0 b and the variation of the rotational constants with v B are consistent with a planar ring equilibrium configuration and an essentially quadratic potential energy function for the ring bending mode. The electric dipole moment components determined from Stark-effect measurements are μ a = 4.12 ± 0.03 D and μ b = 1.71 ± 0.08 D leading to a μ Total = 4.46 ± 0.05 D. A computer-controlled microwave spectrometer is also presented.

Dihydropyran ring conformations. I. Structures of 2-methoxy- and 2-hydroxy-2,4-dimethyl-3,4-dihydro-2H,5H-pyrano[3,2-c][1]benzopyran-5-ones

Conformations of embedded 3,4-dihydro-2H-pyrans (DHP's) are studied in a closely related series of nine 0 I 08· 7681.91/040498-09$03.00 molecular structures, including the cis and trans isomers of both racemic and resolved homologs of 2,4-dimethyl-3,4-dihydro-2H,5H-pyrano[3,2-c][l]· benzopyran-5-one. DHP rings in these structures © 1991 International Union of Crystallography EDWARD J. VALENTE AND DRAKES. EGGLESTON 499 display a range of conformations near the half-chair form but distorted variously toward each of the two inequivalent d,eand e,f-diplanar forms. Factor analysis reveals the coordination of torsional motions associated with a principal ring-bending mode based on the various ring conformations. Two methyl ketals and an hemiketal structure are reported. (I): (2S,4S)-cis-2-methoxy, C 15Hu,04, M, = 260·30, trigonal, P322 1, a = 9·579 (5), c = 24·938 (l 5) A (hexagonal axes), V = 1981 ·7 (33) A', Z=6, Dx = 1·309gcrn , A(MoKa) = 0·71073A, μ = 0·884 cm , F(OOO) = 828, T = 295 K, R = 0·040 for 1470 observations. (II): (2R,4S)-trans-2-methoxy, C15H 160 4, M, = 260·30, monoclinic, P2i. a= 9·254 (4), b = 11 ·970 (5), c = 12·736 (5) A, f3 = 105·64 (4) , V = 1358·5 (22) A3, Z = 4 (two molecules per asymmetric unit), Dx = I ·274 g cm 3, A(Mo Ka) = 0·71073 A, μ : 0·860 cm , F(OOO) = 552, T = 295 K, R = 0·043 for 2923 observations. (III): (2S,4S)-cis-2-hydroxy, C 14H 1404 , M, = 246·27, orthorhombic, P21212i. a= 6·816 (3), b = 12·826 (7), c=l3·949(6)A, V=1219·4(17)A, Z=4, Dx= 1·340gcm3, A(MoKa)=0·7l073A, μ= 0·919 cm 1, F(OOO) = 520, T = 293 K, R = 0·046 for 1291 observations.

The effect of pressure on the Raman spectra of solids. VII. The internal Raman bands in solid and coordinated pyridine

The pressure dependences of the internal Raman-active modes of solid pyridine and pyridine-d5 in both the crystalline and glassy modifications as well as of the complexes Zn(py)2Cl2, Ni(py)2Cl2, and Ni(py)4Cl2 are reported. When pyridine is frozen by the application of pressure, some ring modes as well as those involving the hydrogen atoms reflect this transformation. Upon the coordination of pyridine to metal ions, the ring vibrations show appreciable blue shifts. The pressure dependences of ν1, the C–C stretching mode, and ν12, the in-plane ring bending mode of the pyridine rings, are discussed in detail. The unusually high d5–h5 isotopic ratio of ν12 and its contrasting pressure dependences in the liquid and condensed phases of pyridine-d5 are explained. The association of pyridine molecules in the condensed phase does not occur through hydrogen bonds and the C–H stretching modes, in particular, show that repulsive intermolecular forces become very significant at higher pressures. The ratio of the intensities Iν12/Iν1 varies linearly with the strength of the M–N bonds in a series of pyridine complexes and a correlation also exists between Iν12/Iν1 and ∂ν12/∂p. The vibrations ν1 and ν2 are coupled through Fermi resonance in pyridine and its complexes and the pressure dependence of the Fermi resonance constant W is calculated for Zn(py)2Cl2. The C–H stretching modes reflect the presence of more than one distinct pyridine group in the lattice and are of much lower intensity than in complexes where only one distinct pyridine group is found.

Spectroscopic characterization of anionic tris( N, N-dialkyldithiocarbamate) complexes of zinc(II) and cadmium(II)

The infrared and Raman spectra (1500-50 cm −1) of the related complexes Zn(S 2CNMe 2) 2( I), [Bu 4N][Zn(S 2CNMe 2) 3] ( II), Cd(S 2CNMe 2) 2 ( III), [Bu 4N][Cd(S 2CNMe 2) 3]( IV), Cd(S 2CNEt 2) 2( V) and [Bu 4N][Cd(S 2CNEt 2) 3] ( VI) have been determined. For complexes with coordination number four, metalsulfur stretching frequencies were assigned at 228 cm −1 for I (ZnS), 180 cm −1 for III (CdS) and 208 cm −1 for V (CdS), whereas for complexes with apparent coordination number six they were assigned at 206 cm −1 for II (ZnS) and in the 150–125 cm −1 range for IV and VI (CdS). Thus, expansion of the coordination sphere of the Zn(II) and Cd(II) metal results in a significant shift towards lower energy of the far IR MS vibrations. Other MS sensitive vibrations in the far IR are found in the 385–350 cm −1 range, and are described as mixed ring bending modes with significant MS stretching contribution.

ChemInform Abstract: The Vibrational Spectra (100‐1600 cm‐1) and Scaled ab initio STO‐3G and 3‐21G Harmonic Force Fields for Norbornane, Norbornene, and Norbornadiene.

The vibrational spectra in the region of 100–1600 cm−1 of norbornane and norbornadiene are reinvestigated, and those of norbornene reported for the first time. On the basis of 3‐21G ab initio force fields, evaluated for each molecule and scaled using overlay refinements of 12 scaling factors, the spectra are assigned and the observed transition frequencies reproduced with an overall average error of 6.2 cm−1. STO‐3G force constants, modified to correct for deficiencies in the stretch/bend interaction force constants but otherwise scaled analogously to 3‐21G, reproduce the final set of assignments to within 10.1 cm−1 on average. The STO‐3G basis is shown to be suitable as a less costly alternative to split‐valence basis sets, particularly for describing the vibrational dynamics of low frequency ring bending and torsional modes.

ChemInform Abstract: Effects of Conjugation on the nπ* States of Cyclic Ketones: 3‐Cyclopenten‐1‐one and 2‐Cyclopenten‐1‐one

Abstract The n π * vapor absorption spectra of 3-cyclopenten-1-one (3CP) and 2-cyclopenten-1-one (2CP) and its 5,5- d 2 derivative are studied and compared. The 3CP spectrum, with origin at 30 248 cm −1 , is similar to that of cyclopentanone, except for the more restrictive selection rules imposed by the higher ( C 2 v ) symmetry. The CO bond lengthens by about 0.13Aupon excitation, and the carbonyl group becomes pyramidal with an out-of-plane angle of approximately 32° and a barrier to inversion of 780 cm −1 . Two b 1 and one b 2 modes serve as vibronic origins. The a 2 ring twisting vibration is also active, but only in combination with odd quanta of the b 1 CO wagging mode. A large increase in the frequency of the b 1 ring bending mode upon excitation is rationalized. 2CP, with ( d 0 / d 2 ) origin at 27 211/27 206cm −1 , resembles trans -acrolein which has the same C C C O chromophore. Progressions are observed in the C C O bending (346/337), C O stretching (1355/1356), and C C stretching modes (1416/1416cm −1 ) as well as in other a ′ ring modes. The carbonyl group remains planar in the excited state. Comparisons to 3CP show that many differences between their spectra are the result of conjugation. Previous suggestions that the ring twisting mode in 2CP has a very low frequency and that there is Fermi resonance between the 30 3 and 28 1 levels are substantiated. The ring bending potential, almost harmonic in the ground state, has a very small barrier to planarity in the excited state.

The vibrational spectra (100–1600 cm−1) and scaled a b i n i t i o STO-3G and 3-21G harmonic force fields for norbornane, norbornene, and norbornadiene

The vibrational spectra in the region of 100–1600 cm−1 of norbornane and norbornadiene are reinvestigated, and those of norbornene reported for the first time. On the basis of 3-21G ab initio force fields, evaluated for each molecule and scaled using overlay refinements of 12 scaling factors, the spectra are assigned and the observed transition frequencies reproduced with an overall average error of 6.2 cm−1. STO-3G force constants, modified to correct for deficiencies in the stretch/bend interaction force constants but otherwise scaled analogously to 3-21G, reproduce the final set of assignments to within 10.1 cm−1 on average. The STO-3G basis is shown to be suitable as a less costly alternative to split-valence basis sets, particularly for describing the vibrational dynamics of low frequency ring bending and torsional modes.

Effects of conjugation on the nπ* states of cyclic ketones: 3-Cyclopenten-1-one and 2-cyclopenten-1-one

The nπ* vapor absorption spectra of 3-cyclopenten-1-one (3CP) and 2-cyclopenten-1-one (2CP) and its 5,5-d2 derivative are studied and compared. The 3CP spectrum, with origin at 30 248 cm−1, is similar to that of cyclopentanone, except for the more restrictive selection rules imposed by the higher (C2v) symmetry. The CO bond lengthens by about 0.13Åupon excitation, and the carbonyl group becomes pyramidal with an out-of-plane angle of approximately 32° and a barrier to inversion of 780 cm−1. Two b1 and one b2 modes serve as vibronic origins. The a2 ring twisting vibration is also active, but only in combination with odd quanta of the b1 CO wagging mode. A large increase in the frequency of the b1 ring bending mode upon excitation is rationalized. 2CP, with (d0/d2) origin at 27 211/27 206cm−1, resembles trans-acrolein which has the same CCCO chromophore. Progressions are observed in the CCO bending (346/337), CO stretching (1355/1356), and CC stretching modes (1416/1416cm−1) as well as in other a′ ring modes. The carbonyl group remains planar in the excited state. Comparisons to 3CP show that many differences between their spectra are the result of conjugation. Previous suggestions that the ring twisting mode in 2CP has a very low frequency and that there is Fermi resonance between the 303 and 281 levels are substantiated. The ring bending potential, almost harmonic in the ground state, has a very small barrier to planarity in the excited state.

Microwave spectrum, dipole moment and conformation of 5,6-dihydro-2H-pyran-2-one

The microwave spectrum of 5,6-dihydro-2H-pyran-2-one has been recorded from 18.0 to 40.0 GHz. Both a-type and b-type R-branch transitions in the ground state as well as a-type transitions in the first three excited states of the right-bending and the first excited state of the ring-deforming mode are assigned. The ground state effective rotational constants are determined to be A = 5143.00 ± 0.04, B = 2703.14 ± 0.02 and C = 1864.29 ± 0.01 MHz. Analysis of the measured Stark effects gives the dipole moment 5.08 ± 0.04 D with components of |μ a | = 4.96 ± 0.02, |μ b | = 1.01 ± 0.09 D and |μ c | = 0.45 ± 0.01 D. It is shown that the ring skeleton is non-planar from the magnitude of the μ c component of the dipole moment as well as from the value of I a + I b − I c . From the experimental data, the ring bending mode appears to be governed by a double minimum potential function with a barrier to the planarity larger than 250 cm −1.

ChemInform Abstract: CONFORMATION AND BARRIER TO INTERNAL ROTATION OF 3‐METHYL‐2‐CYCLOPENTEN‐1‐ONE BY MICROWAVE SPECTROSCOPY

Abstract The microwave spectrum of 3-methyl-2-cyclopenten-1-one has been recorded from 18.0 to 26.5 GHz. Both a -type and c -type R -branch transitions in the ground state and the three vibrationally excited states of the ring-bending mode have been assigned. The ground state rotational constants are determined to be A = 5799.78 ± 0.23, B = 2028.79 ± 0.02, and C = 1546.77 ± 0.01 MHz. These data are consistent with a planar heavy-atom skeleton. From an analysis of the internal rotational splittings in the first methyl torsional state, the threefold barrier for the methyl group has been determined to be 1.71 ± 0.09 kcal mol −1 .

Microwave, infrared and Raman spectra of 1-fluorocyclohexene

The microwave spectrum of 1-fluorocyclohexene has been recorded from 18.0 to 39.5 GHz. Only a-type R-branch transitions were observed and assigned for the ground and three vibrationally excited states. The ground state rotational constants are: A = 4557.16 ± 0.08, B = 2297.77 ± 0.01, C = 1630.41 ± 0.01 MHz. It is shown that the quantity I a + I b − I c = 20.87 U·Å 2 cannot arise totally from the eight hydrogens on the four CH 2 groups but that the heavy atom skeleton contributes to it, which indicates that the heavy atom skeleton is nonplanar. The rotational constants are shown to be consistent with reasonably assumed heavy atom structural parameters in the half chair configuration with a dihedral angle of puckering of approximately 30°. From the Stark effect the dipole moment components were determined to be: ∥μ a ∥ = 1.886 ± 0.012, ∥μ b ∥ = 0.463 ± 0.031 and ∥μ c ∥ ≅ 0 with ∥μ t ∥ = 1.942 ± 0.001 D. From relative intensity measurements it was estimated that the first, second and third excited vibrational states would occur at frequencies of 150 ± 15,234 ± 20 and 282 ± 20 cm −1, respectively. The i.r. (3500-80 cm −1) spectra of the gas and liquid and the Raman spectrum (3500-100 cm −1) of the liquid have been recorded. All of the normal modes have been assigned based on i.r. band contours, relative intensities and group frequencies. The ring-twisting and ring-bending modes have been assigned to i.r. bands at 239.8 and 139.3 cm −1, respectively, and both modes have associated “hot bands” falling to lower frequencies. From these progressions the barrier to interconversion is estimated to be in the range 2500–4400 cm −1 (7.15–12.6 kcal/mol) with a barrier to planarity in the range 7600–14,000 cm −1 (21.6–40.0 kcal/mol). These results are compared to the corresponding quantities in some similar molecules.

Conformation and barrier to internal rotation of 3-methyl-2-cyclopenten-1-one by microwave spectroscopy

The microwave spectrum of 3-methyl-2-cyclopenten-1-one has been recorded from 18.0 to 26.5 GHz. Both a-type and c-type R-branch transitions in the ground state and the three vibrationally excited states of the ring-bending mode have been assigned. The ground state rotational constants are determined to be A = 5799.78 ± 0.23, B = 2028.79 ± 0.02, and C = 1546.77 ± 0.01 MHz. These data are consistent with a planar heavy-atom skeleton. From an analysis of the internal rotational splittings in the first methyl torsional state, the threefold barrier for the methyl group has been determined to be 1.71 ± 0.09 kcal mol −1.

Excited state amino inversion potentials in aniline derivatives: Fluoroanilines and aminopyridines

Abstract The near u.v. vapour absorption spectra of 2-, 3- and 4-fluoroaniline, 2- and 3-aminopyridine, and their -ND2 derivatives have been measured. Although all these molecules, like aniline, have pyramidal amino groups in their ground states, there is no evidence for non-planarity in the S1 excited states. The observed amino inversion levels of aniline, the fluoroanilines, and probably 2-aminopyridine for which the data are less complete, can be fitted to a zero-barrier potential V = hcν0[ 1 2 q2 + αq4]. There is evidence that one of the out-of-plane ring bending modes in 2-fluoroaniline decreases in frequency from ca. 440 to less than 50 cm−1 and becomes anharmonic in the excited state.

Ring-bending potential function and electric dipole moment of maleic anhydride

A vibrational satellite progression of the ring-bending vibration ( B 1) of the C 2 v molecule maleic anhydride has been assigned in the microwave spectrum measured in the region 18.0–26.5 GHz. The rotational constants for four vibrational excited states of the ring-bending mode were determined. From the vibrational separation as determined from the inertial defect variation a potential function governing the ring bending motion was found to be of the form V ( Z) = 7.01 ( Z 4 + 16.55 Z 2), where Z is the reduced ring-bending coordinate. This potential function is in good agreement with those obtained from the variation of the rotational constants with vibrational state. The electric dipole moment was determined to be 4.141 ± 0.022 D for the ground state. The dipole moment for the first two excited states was also determined.

ChemInform Abstract: MICROWAVE SPECTRUM OF CYCLOHEXANONE

Abstract The rotational spectrum of cyclohexanone has been observed within the frequency region from 18.0 to 40.0 GHz. Transitions in the ground state and six excited states have been assigned. The ground state rotational constants are (in MHz) A = 4195.316 +- 0.059, B = 2502.627 ± 0.005 and C = 1754.443 ± 0.005. From information obtained from relative intensity measurements, these excited states are estimated to be ~ 100 cm −1 above the ground state for the first excited state of the ring-bending mode and ~ 180 cm −1 for the first excited state of the ring-twisting mode.

Microwave spectrum, conformation, and dipole moment of 4-thiacyclohexanone

The microwave spectra of 4-thiacyclohexanone in the ground state and eight vibrationally excited states have been studied in the frequency region 18.0–40.0 GHz and the corresponding rotational constants have been determined. The following values of the ground-state rotational constants (MHz) were obtained from the analysis of the a-type transitions: A = 3935.149 (0.031), B = 1829.444 (0.001), and C = 1364.609 (0.001). Analysis of the Stark effect gives for the dipole components (in Debye units) μa = 1.409 (0.002), μc = 0.391 (0.064). These data are consistent with a chair conformation for the ring. A phisically reasonable set of structural parameters which reproduce the ground-state rotational constants has been derived. A qualitative estimate of the low-frequency vibrational modes was obtained from relative-intensity measurements. The lowest vibrational frequency is believed to be a ring-bending mode and it occurs at 77 ± 22 cm−1 while the ring-twisting mode is at 204 ± 27 cm−1.

Microwave spectrum of cyclohexanone

The rotational spectrum of cyclohexanone has been observed within the frequency region from 18.0 to 40.0 GHz. Transitions in the ground state and six excited states have been assigned. The ground state rotational constants are (in MHz) A = 4195.316 +- 0.059, B = 2502.627 ± 0.005 and C = 1754.443 ± 0.005. From information obtained from relative intensity measurements, these excited states are estimated to be ~ 100 cm −1 above the ground state for the first excited state of the ring-bending mode and ~ 180 cm −1 for the first excited state of the ring-twisting mode.

The ring puckering potential function of trimethylene sulfide and trimethylene selenide

The rotational constants A, B and C are functions of the vibrational quantum numbers for the out-of-plane bending modes of trimethylene sulfide and trimethylene selenide. Infrared and microwave spectroscopy have been used to determine the dependence between these parameters. The height and shape of the barrier arising from combination of the ring bending modes is also determined. CNDO/2 calculations are used to predict the position of the hydrogen atoms of the methylene groups in trimethylene sulfide and trimethylene selenide. The four-membered rings are non-planar for several vibrational states of the ring bending modes.

Microwave rotational spectrum, ring-puckering vibrations, ring conformation and electric dipole moment of γ-crotonolactone

The rotational spectrum of γ-crotonolactone has been observed and measured in the frequency range 17.0–40.0 GHz. R-Branch and Q-branch transitions allowed by µa and µb have been assigned for the vibrational ground state, the excited states vB= 1, 2 and 3 of the ring-bending mode and the excited state vT= 1 of the ring-twisting mode. Relative intensity measurements among vibrational satellites give the vibrational separations vB= 1 â†� 0 and vT= 1 â†� 0 as 199 and 349 cm–1, respectively. The electric dipole moment components determined from Stark-effect measurements are µa= 4.726 ± 0.01 D, µb= 1.718 ± 0.004 D and µc= 0.0 D, leading to µTOTAL= 4.905 ± 0.011 D. Consideration of the quantity Δ0=I0c–I0b–I0a, the variation of the rotational constants with vB, vibrational separations and the value µc= 0.0 D confirms that the heavy atoms of γ-crotonolactone are strictly coplanar at equilibrium and shows that the mode νB is governed by an essentially quadratic potential-energy function.

ChemInform Abstract: THE ROTATIONAL AND VIBRATIONAL SPECTRA, STRUCTURE AND DIPOLE MOMENT OF 1,3‐DIOXOLE‐2‐THIONE

Abstract Rotational and centrifugal distortion constants are determined for the ground state and several excited states of the lowest vibrational mode of l,3-dioxole-2-thione, the “envelope” ring-bending mode. The structure of the molecule is planar. The A-constant is 1% larger than that of 1,3-dioxole-2-one (vinylene carbonate), which indicates small changes in ring-geometry on substituting CS for CO. The dipole moment of 1,3-dioxole-2-thione deduced from Stark-effect measurements is (1.60 ± 0.02) × 10 −29 Cm (4.81 ± 0.06 D). IR and Raman spectra lead to a complete analysis of the normal modes of vibration, based partially on a normal coordinate calculation.