Equatorial Chair Conformers Research Articles

Reversible Self-Assembly of an N-Heterocyclic Carbene on Metal Surfaces.

Self-assembly of cyclohexyl cyclic (alkyl)(amino)carbenes (cyCAAC) can be realized and reversibly switched from a close-packed trimer phase to a chainlike dimer phase, enabled by the ring-flip of the cyclohexyl wingtip. Multiple methods including scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations identified a distinct isomer (axial or equatorial chair conformer) in each phase, and consequently support the conclusion regarding the determination of molecular surface geometry on the self-assembly of cyCAAC. Moreover, various substrates such as Ag (111) and Cu (111) are tested to elucidate the importance of cyCAAC-surface interactions on cyCAAC based nanopatterns. These investigations of patterned surfaces prompted a deep understanding of cyCAAC binding mode, surface geometry and reversible self-assembly, which are of paramount significance in the areas of catalysis, biosensor design and surface functionalization.

Reversible Selbstorganisation eines N‐heterocyclischen Carbens auf Metalloberflächen

AbstractCyclohexyl(alkyl)(amino)carbene (cyCAAC) bilden selbstorganisierende Monolagen, die durch den Ring‐Flip der Cyclohexyl‐Seitengruppe reversibel von einer dicht gepackten Trimer‐Phase in eine kettenartige Dimer‐Phase umgeschaltet werden können. Mehrere Methoden, darunter Rastertunnelmikroskopie (STM), Röntgen‐Photoelektronenspektroskopie (XPS) und Dichtefunktionaltheorie (DFT)‐Berechnungen, identifizierten ein eindeutiges Isomer (axiales oder äquatoriales Sesselkonformer) in jeder Phase und stützen somit die Schlussfolgerung hinsichtlich des Einflusses der molekularen Oberflächengeometrie auf die Selbstorganisation von cyCAAC. Darüber hinaus wurden verschiedene Substrate wie Ag(111) und Cu(111) getestet, um die Bedeutung der Wechselwirkungen zwischen cyCAAC und der Oberfläche für die auf cyCAAC basierenden Nanomuster aufzuklären. Die Untersuchungen von gemusterten Oberflächen führten zu einem tieferen Verständnis der cyCAAC‐Bindungsweise, der Oberflächengeometrie und der reversiblen Selbstorganisation, die in den Bereichen Katalyse, Biosensor‐Design und Oberflächenfunktionalisierung von herausragender Bedeutung sind.

Conformational analysis of morpholine studied using Raman spectroscopy and density functional theoretical calculations

Morpholine is a very interesting cyclic compound which is an ether as well as a secondary amine. The relative distribution of different conformations of morpholine depends on the medium. Using Raman spectroscopy and theoretical calculations, we found that, equatorial chair conformer is predominant in the pure liquid, but in aqueous solution, contribution from the axial conformer increases. The surface-enhanced Raman scattering (SERS) studies in presence of silver nanoparticles showed change in relative intensities of different vibrational modes of morpholine. It was found that the axial chair conformer is preferentially adsorbed vertically through the NAg bond on the nanoparticle surface.

Conformational analysis of 3-methyltetrahydro-1,3-oxazine

Conformational analysis of 3-methyltetrahydro-1,3-oxazine has been performed using HF/6-31G(d), PBE/3z, and RI-MP2/λ2 simulation approximations. The potential energy surface contains eight minima. Interconversion of the axial and equatorial chair conformers (main minima) occurs via several independent pathways involving six twist forms. The preferred path presumes direct chair-chair transition via pyramidal inversion of the nitrogen atom.

Conformational analysis of 5-methyl- and 2,2,5-trimethyl-1,3-dithianes

Conformational isomerization of 5-methyl- and 2,2,5-trimethyl-1,3-dithianes was studied using quantum-chemical HF/6-31G(d), HF/pVDZ, and PBE/3z approximations. The potential energy surface of molecules of both compounds is shown to contain the main minimum corresponding to the equatorial chair conformer C-5e. The calculated potential barriers of the conformational isomerization were found. Based on the experimental (1H NMR) and theoretical vicinal coupling constants the ΔG 0 values were determined of the methyl group at the ring C5 atom. The reasons for the distinctions between the values of this parameter from the ΔG 0, obtained by energy minimizations of the equatorial and axial chair conformers are discussed.

Vacuum ultraviolet mass-analyzed threshold ionization spectroscopy of methylcyclohexane in the supersonic jet

Vacuum ultraviolet (VUV) mass-analyzed threshold ionization (MATI) spectrum of supersonically cooled methylcyclohexane has been obtained to give the precise adiabatic ionization energy of 9.6958±0.0025eV for the chair equatorial conformer. Vibrationally resolved MATI spectrum has been analyzed with the aid of density functional theory and Franck–Condon calculations. The MATI spectrum reflects the structural change upon ionization and its origin is discussed by inspecting the shapes of the valence orbitals involved in the ionization process. The spectroscopic implication of the structural interconversion above the certain energy level is discussed with theoretical calculations of molecular structures and energetics.

Theoretical vibrational spectra compared to the experiment and the anomeric effect in 2-chloro-1,3,2-dioxaphosphorinane-2-oxide, -sulfide, and -selenide. II. Vibrational spectra and assignments

2-Chloro-1,3,2-dioxaphosphorinane-2-oxide, -sulfide, and -selenide are studied with the help of DFT/B3LYP and several ab initio methods using a 6-311G** basis set. However, due to rather large relative energies of higher conformers in all three cases, the conformational equili-brium mixture contains more than 95% (see the preceding paper in this Journal) of the lowest chair-equatorial conformer (this indicates that the P=X bond is in the equatorial position), so we do not find any conformer bands in the experimental spectra and calculate our theoretical ones for the assignment only from the chair-equatorial conformer. The vibrational infrared and Raman spectra were calculated and are in fair agreement with their experimental counterparts. Potential energy distribution calculations are performed, and the theoretical modes where an experimental counterpart could be found to symmetry coordinates are assigned.

Anomeric effect and theoretical vibrational spectra compared to the experiment in 2-chloro-1,3,2-dioxaphosphorinane-2-oxide, -sulfide, and -selenide. I. Anomeric effect and structures

2-Chloro-1,3,2-dioxaphosphorinane-2-oxide, -sulfide, and -selenide are studied at the DFT/B3LYP level and several ab initio methods using a 6-311G** basis set. Our energy optimizations by all these methods show that for oxide DFT and ab initio methods are not much different, while for the sulfide and the selenide the DFT relative energies are higher by about a kcal/mol as compared to those of MP2, MP3, MP4(SDTQ)//MP2, and CCSD(T)//MP2 (//MP2 indicates that a single-point calculation based on the MP2 optimized geometry is performed). However, regardless of rather large relative energies, that does not change the fact that in all three cases the conformational equilibrium mixture contains more than 95% of the lowest, chair-equatorial conformer (this indicates that the P=X bond is in equatorial position). This one and the next higher conformer (chair-axial) are confirmed to be real conformers (energy minima) in all cases. The energetically much higher twist and boat forms are probably just stationary states and local maxima because in many cases, geometry optimizations do not converge to them. Only for MP2 and the selenide do all optimizations converge to the desired stationary state. The relative energies could all be explained in terms of anomeric effects and ring strains. The decreasing covalent character of the P=X bond, with X changing from O to S and to Se, shows itself in the increasing bond lengths and the decreasing strength of anomeric effects.

Microwave, infrared, and Raman spectra, r0 structural parameters, conformational stability, and vibrational assignment of ethynylcyclohexane

The FT-microwave spectrum of ethynylcyclohexane, c-C 6H 11CCH has been investigated from 11,000 to 21,000 MHz and 32 transitions for the chair-equatorial conformer and 22 transitions for the chair-axial form have been observed and assigned from which the rotational constants and five centrifugal distortion constants have been obtained. By utilizing the six microwave rotational constants along with ab initio MP2(full)/6-311 + G(d,p) predicted structural values, adjusted r 0 parameters have been obtained for the two conformers. The determined heavy atom structural parameters for the equatorial[axial] conformer are: the distances (Å) C 1 C 9,10 = 1.532(3)[1.537(3)], C 9,10 C 15,16 = 1.544(3)[1.534(3)], C 4 C 15,16 = 1.541(3)[1.545(3)], C 6 C 7 = 1.210(3)[1.212(3)] and angles in degrees: ∠C 1C 9,10C 15,16 = 111.0(5)[111.2(5)], ∠C 6C 4C 15,16 = 110.7(5)[110.3(5)] and τC 1C 9,10C 15,16C 4 = 56.4(10)[55.3(10)]. Variable temperature (−60 to −100 °C) studies of a xenon solution were carried out and by using 10 conformer pairs an enthalpy difference of 124 ± 13 cm −1 (1.48 ± 0.16 kJ/mol) was obtained with the more stable form the chair-equatorial conformer. At ambient temperature, the abundance of the chair-equatorial conformer is 55 ± 3%. By measuring the temperature dependency of the Raman spectrum of the liquid of the conformer pair at 822 (axial)/842 (equatorial) cm −1, a standard enthalpy difference of 172 ± 32 cm −1 (2.06 ± 0.38 kJ/mol) was obtained again with the chair-equatorial conformer the more stable form. From MP2(full)/6-311 + G(d,p) ab initio calculations the chair-axial conformer is predicted to be more stable by 135 cm −1 (1.62 kJ/mol) whereas in contrast the density functional theory calculations by the B3LYP method with the same basis set gives the chair-equatorial conformer more stable by 289 cm −1 (3.46 kJ/mol). A complete vibrational assignment is given for the chair-equatorial conformer and most of the fundamentals for the chair-axial form have been assigned. To support the vibrational assignments, normal coordinate calculations with two scaling factors for the force constants from MP2(full)/6-31G(d) calculations were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values and infrared band contours. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.

Conformational stability from variable temperature infrared spectra of xenon solution, r0 structural parameters, and vibrational assignment of fluorocyclohexane

The infrared spectra (3500–300 cm −1) of the gas and xenon solutions of fluorocyclohexane have been recorded. By the utilization of nine different temperatures of nine conformer pairs the enthalpy difference between the more stable chair-equatorial conformer and the chair-axial form was determined to be 48 ± 5 cm −1 (137 ± 14 cal/mol). The ab initio predicted values ranged from 121 cm −1 with the chair-axial conformer more stable from the MP2(full)/6-311G(d,p) calculations to 24 cm −1 with the chair-equatorial form more stable with diffuse functions. The r 0 structural parameters have been obtained from predicted parameters from ab initio MP2(full)/6-311+G(d,p) calculations adjusted to fit the previously reported microwave determined rotational constants. The determined heavy atom structural parameters for the equatorial[axial] conformer are: the distances (Å) C 1–C 7,8 = 1.533(3)[1.531(3)], C 7,8–C 13,14 = 1.543(3)[1.538(3)], C 4–C 13,14 = 1.518(3)[1.520(3)], C 4–F 6 = 1.404(3)[1.410(3)] and angles in degrees ∠C 1C 7,8C 13,14 = 111.0(5)[111.2(5)], ∠F 6C 4C 13,14 = 108.7(5)[107.9(5)] and τC 8C 1C 7C 13 = 55.9(10)[55.9(10)]. The harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies have been obtained for both conformers from MP2(full)/6-31G(d) ab initio calculations. With two scaling factors of 0.88 for the C–H stretches and 0.9 for the remaining ones, the fundamental wavenumbers have been predicted and along with the depolarization values and infrared band contours (B-type for A″ modes) a complete vibrational assignment has been made for both conformers. The results are discussed and compared to the corresponding properties of some similar molecules.

Microwave, Raman, and infrared spectra, r0 structural parameters, conformational stability, and vibrational assignment of cyanocyclohexane

The FT-microwave spectrum of cyanocyclohexane, c-C 6H 11CN has been investigated from 11,000 to 21,000 MHz and 99 transitions for the chair-equatorial conformer and 76 transitions for the chair-axial form have been observed and assigned from which the rotational constants and five centrifugal distortion constants have been obtained. By utilizing the six microwave rotational constants along with ab initio MP2(full)/6-311 + G(d,p) predicted structural values, adjusted r 0 parameters have been obtained for the two conformers. The determined heavy atom structural parameters for the axial[equatorial] conformer are: the distances (Å) C 1–C 8,9 = 1.536(3)[1.536(3)], C 8,9–C 14,15 = 1.532(3)[1.532(3)], C 4–C 14,15 = 1.544(3)[1.544(3)], C 6 N 7 = 1.162(3)[1.161(3)] and angles in degrees: ∠C 1C 8,9C 14,15 = 111.6(5)[110.8(5)], ∠C 6C 4C 14,15 = 110.0(5)[110.8(5)] and τC 1C 8,9C 14,15C 4 = 55.6(10)[56.9(10)]. Variable temperature (−55 to −100 °C) studies of a xenon solution were carried out and by using two conformer pairs from which an enthalpy difference of 63 ± 9 cm −1 (0.75 ± 0.11 kJ/mol) was obtained with the more stable form the chair-axial conformer. At ambient temperature, the abundance of the chair-axial conformer is 58 ± 8%. By measuring the temperature dependency of the Raman spectrum of the liquid of the conformer pair at 819 (axial)/842 (equatorial) cm −1, a standard enthalpy difference of 46 ± 16 cm −1 (0.54 ± 0.19 kJ/mol) was obtained again with the chair-axial conformer the more stable form. From MP2(full)/6-311 + G(d,p) ab initio calculations the chair-axial conformer is predicted to be more stable by 120 cm −1 (1.44 kJ/mol) whereas in contrast the density functional theory calculations by the B3LYP method with the same basis sets gives the chair-equatorial conformer more stable by 180 cm −1 (2.16 kJ/mol). A complete vibrational assignment is given for the chair-axial conformer and most of the fundamentals for the chair-equatorial form have been assigned. To support the vibrational assignments, normal coordinate calculations with two scaling factors for the force constants from MP2(full)/6-31G(d) calculations were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values and infrared band contours. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.

Ab Initio conformational analysis of tetrahydro-1,3-oxazine

Nonempirical RHF/STO-3G and 6-31G(d) studies on conformational behavior of tetrahydro-1,3- oxazine showed that interconversion between the axial (global minimum) and equatorial chair conformers can follow five independent pathways. The potential energy surface contains seven minima corresponding to chair and twist conformers, as well as seven transition states having sofa, half-chair, and symmetrical and unsym- metrical boat conformations. Additional potential barriers to interconversion, resulting from pyramidal inversion of the nitrogen atom, were also revealed.

The conformer resolved ultraviolet photodissociation of morpholine

Distinct sets of resolved photoproducts arising from the photodissociation of two neutral molecule conformers have been observed using H(Rydberg) atom photofragment translational spectroscopy methods. This study focuses on the photodissociation of morpholine, at many wavelengths between 250 nm and 193 nm. Morpholine, a six-membered saturated heterocycle with the molecular formula HN(CH2CH2)2O, exists as two ground state chair conformers that are distinguished by the orientation of the N–H bond (axial or equatorial) relative to the chair ring. Complementary MP2, QCISD and DFT calculations all predict the equatorial chair conformer to be lower in energy by ∼250 cm−1. CASPT2(8/8) calculations confirm that both conformers have similar electronic structures, and share a common first excited singlet state, arising as a result of electron promotion from the nitrogen lone pair orbital (n) to the 3s/σ* orbital centred on the N–H bond, forming a 1nσ* state. Photoexcitation to the 1nσ* potential energy surface (PES) leads to N–H bond extension, with the dissociating flux passing through a conical intersection with the ground state PES en route to morpholinyl radicals and H atom photoproducts. Careful analysis of the resulting H atom time-of-flight spectra reveals peaks attributable to the population of specific vibrational states of morpholinyl products arising from dissociation of both conformers, enabling determination of the energy difference between the two ground conformers of morpholine as 180 ± 50 cm−1, and the N–H bond strengths in both the axial and equatorial conformers as 33200 ± 50 cm−1 and 33380 ± 50 cm−1, respectively.

Conformational isomerization of 4-methyl-1,3-dioxane

The whole possible course of conformational isomerization of 4-methyl-1,3-dioxane has been established using empirical (MM+) and nonempirical [STO-3G, 3-21G, 6-31G(d) and 6-31G(d,p)] approximations within the limits of the Hartree-Fock method. It was shown that the potential energy surface of this compound contains a principal (equatorial chair conformer) and local minima corresponding to the axial chair conformer and series flexible forms.

Adsorption changes of cyclohexyl isothiocyanate on gold surfaces

The adsorption and structure of cyclohexyl isothiocyanate (CHIT) on gold surfaces has been investigated by surface-enhanced Raman scattering (SERS) and scanning tunneling microscopy (STM). Depending on the concentration, the spectral changes of the NCS stretching vibration on gold nanoparticles appeared to be more conspicuous than those of cyclohexyl ring modes. Both equatorial and axial chair conformers of CHIT were found to exist at low bulk concentrations near the monolayer coverage limit, whereas the equatorial chair conformer appeared to be dominant at high bulk concentrations. It was also observed that the ring conversion of equatorial to axial conformers can easily occur at higher temperatures, and the mole fraction of the axial form is assumed to increase with increasing temperature from 30 to 60 °C. Alternatively, STM imaging revealed that the adsorption of CHIT molecules on Au(1 1 1) leads to the formation of disordered SAMs with a few vacancy islands, as opposed to the formation of well-ordered self-assembled monolayers (SAMs) by cyclohexanethiols.

Microwave spectra, r0 structural parameters, and conformational stability from xenon solutions of silylcyclohexane

The FT-microwave spectrum of silylcyclohexane, c-C 6H 11SiH 3 has been investigated from 11,000 to 21,000 MHz and 43 transitions for the 28Si isotopomer have been observed and assigned for the chair-equatorial conformer and likewise for 23 and 18 transitions for the 29Si and 30Si isotopomers, respectively. The SiH a and SiH s distances have been determined from their stretching frequencies with values of 1.486 and 1.487 Å, respectively. With these parameters and by utilizing ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r 0 have been obtained. The determined heavy atom structural parameters for the most stable chair-equatorial conformer are: the distances (Å) r 0(C 1–C 10, 11) = 1.534(3); r 0(C 10, 11–C 16, 17) = 1.530(3); r 0(C 4–C 16, 17) = 1.544(3); r 0(C 4–Si 6) = 1.880(5) and the angles in degrees: C 1C 10, 11C 16, 17 = 111.1(5)°; Si 6C 4C 16, 17 = 111.6(5)° with the dihedral angle C 1C 10,11C 16,17C 4 = 56.0(10)°. Variable temperature (−55 to −100 °C) studies of the infrared spectra (4000 to 400 cm −1) of silylcyclohexane dissolved in liquefied xenon have been carried out as well as the infrared spectra of the gas and solid. By measuring the temperature dependency of three conformer pairs from the spectra of xenon solutions, a standard enthalpy difference of 414 ± 19 cm −1 (1.18 ± 0.05 kcal/mol) was obtained with the chair-equatorial conformer the more stable form. At ambient temperature, the abundance of the axial conformer is 12.7 ± 1%. From MP2 ab initio calculations utilizing various basis sets, the equatorial conformer is predicted to be more stable by 525 ± 10 cm −1 from the four largest basis set calculations, which is consistent with experimental results. However, the average from the corresponding B3LYP density functional theory calculations is 671 ± 5 cm −1 which is certainly too large. A few of the previously reported vibrational assignments have been corrected. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.

Concentration‐ and temperature‐dependent conformation change of cyclohexyl isocyanide on gold nanoparticle surfaces

AbstractThe conformational changes of cyclohexyl isocyanide (CHNC) on gold nanoparticle surfaces were investigated by means of concentration‐ and temperature‐dependent surface‐enhanced Raman scattering (SERS). The equatorial chair conformer appeared to be dominant at high‐bulk concentrations or low temperatures, whereas both the equatorial and axial chair conformers of CHNC were found to exist at low‐bulk concentrations or high temperatures as in the previous reports of cyclohexanethiol (CHT). Depending on concentrations and temperatures, the spectral changes of the NC stretching vibration on gold nanoparticles appeared to be more conspicuous than those of the cyclohexyl ring modes. A density functional theory (DFT) calculation was performed at the level of B3LYP/6‐31G + + (d,p) to compare the energetic stability of the various conformers of CHT and CHNC. The energy differences between the equatorial and axial chair conformers were predicted to be smaller for CHNC than for CHT by ∼3 kJ mol−1 from the DFT calculation. Copyright © 2008 John Wiley & Sons, Ltd.

Conformational stability of bromocyclohexane from temperature dependent FT-IR spectra of xenon solutions, r0 structural parameters and vibrational assignment

The infrared spectra (3200–30 cm −1) of the gas, amorphous and annealed solid and Raman spectra (3200–30 cm −1) of liquid and solid bromocyclohexane ( c-C 6H 11Br) have been recorded. Variable temperature (−55 to −100 oC) studies of the infrared spectra (3200–400 cm −1) of bromocyclohexane ( c-C 6H 11Br) dissolved in liquefied xenon have been carried out. From these data, the enthalpy difference has been determined to be 239 ± 24 cm −1 (2.86 ± 0.29 kJ mol −1), with the chair-equatorial conformer the most stable form. At ambient temperature, the abundance of the chair-axial conformer is 24% ± 2. From MP2 ab initio calculations utilizing various basis sets with and without diffuse functions, the equatorial conformer is predicted to be more stable by 168 ± 22 cm −1 from the four largest basis set calculations. However, the average from the corresponding B3LYP density functional theory calculations is 300 ± 16 cm −1 which, considering the error, is consistent with the experimental value. By utilizing the previously reported microwave rotational constants for two isotopomers ( 79Br, 81Br) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r 0 structural parameters have been obtained. The determined heavy atom distances for the most stable chair-equatorial conformer in Å are r 0(C 1–C 7, 8) = 1.532(3); r 0(C 7, 8–C 13, 14) = 1.539(3); r 0(C 4–C 13, 14) = 1.524(3); and r 0(C 4–Br 6) = 1.966(5) and the angles in degrees: ∠C 1C 7, 8C 13, 14 = 111.3(5)°; ∠Br 6C 4C 13, 14 = 109.8(5)° with the two dihedral angles ∠C 8C 1C 7C 13 = 55.9(10)° and ∠C 14C 4C 13C 7 = 57.7(10)° .For the axial form the major differences in distances are obtained for the r 0(C 7,8–C 13,14) = 1.531(3) and r 0(C–Br) = 1.975(3) Å. These parameters are in good agreement with those reported earlier from microwave and electron diffraction studies where the CC and CH distances were all assumed to be equal. A few of the previously reported vibrational assignments have been corrected. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.

Conformational stability of chlorocyclohexane from temperature-dependent FT-IR spectra of xenon solutions, r 0 structural parameters, and vibrational assignment

Variable temperature (−55 to −105 °C) studies of the infrared spectra (4000–400 cm−1) of chlorocyclohexane (c-C6H11Cl) dissolved in liquefied xenon have been carried out. The infrared spectra of the gas and solid have also been recorded from 4000–100 cm−1. By analyzing six conformer pairs in the xenon solution, a standard enthalpy difference of 132 ± 13 cm−1 (1.58 ± 0.16 kJ/mol) was obtained with the equatorial conformer the more stable form. At ambient temperature, the abundance of the axial conformer is 34 ± 1%. The potential surface describing the conformational interchange has been determined and the Fourier coefficients were obtained. From MP2 ab initio calculations utilizing various basis sets with and without diffuse functions, the equatorial conformer is predicted to be more stable by 161 ± 18 cm−1 from the four largest basis set calculations, which is consistent with the experimental results. However, the average from the corresponding B3LYP density functional theory calculations is 274 ± 15 cm−1 which is certainly too large. By utilizing the previously reported microwave rotational constants for two isotopomers (35Cl, 37Cl) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r 0 structural parameters have been obtained. The determined heavy atom distances for the most stable chair-equatorial conformer in A are: r 0(C1–C7,8) = 1.532(3); r 0(C7,8–C13,14) = 1.536(3); r 0(C4–C13,14) = 1.524(3); and r 0(C4–Cl6) = 1.802(5) and the angles in degrees: ∠C1C7,8C13,14 = 111.3(5)o; ∠Cl6C4C13,14 = 109.7(5)o with the two dihedral angles ∠C8C1C7C13 = 56.3(10)o and ∠C14C4C13C7 = 56.7(10)o. These parameters are in good agreement with those reported earlier from microwave and electron diffraction studies where the CC and CH distances were all assumed to be equal. A few of the previously reported vibrational assignments have been corrected. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.

Conformational changes of cyclohexanethiol adsorbed on gold surfaces

The conformational changes of cyclohexanethiol (CHT) on gold nanoparticles and gold plate surfaces were investigated by means of surface-enhanced Raman scattering. The equatorial chair conformer appeared to be dominant at high bulk concentrations, whereas, both the equatorial and axial chair conformers of CHT were found to exist at low bulk concentrations corresponding to a near monolayer coverage limit. Molecular-scale scanning tunneling microscopy imaging revealed the existence of two axial and equatorial conformers in the CHT monolayer on an Au(1 1 1) surface as well as the existence of the meta-stable conformer, which may be due to dynamic conformational changes of the cyclic ring. It was also found that the peaks corresponding to the axial bands disappeared below room temperature, indicating the equatorial conformer is dominant at low temperatures. From this result, we found that the conformation of CHT on an Au surface was easily changed by surface temperature.