Universal isotope effect in thermal diffusion of mixtures containing cyclohexane and cyclohexane-d12

The Soret coefficients S(T) of the liquids acetone, benzene, benzene-d1, 1,3,5-benzene-d3, benzene-d5, benzene-13C6, benzene-d6, n-hexane, toluene, 1,2,3,4-tetrahydronaphthalene, isobutylbenzene, and 1,6-dibromohexane have been measured in protonated and perdeuterated cyclohexane by a transient holographic grating technique. It has been found that S(T) can be either positive or negative and even change its sign as a function of concentration. The isotope effect DeltaS(T)=-0.99 x 10(-3) K(-1), which is the change of S(T) after isotopic substitution of cyclohexane, neither depends on concentration nor on the nature of the mixing partner. Only in the case of the polar acetone is DeltaS(T) approximately 30% larger but still concentration independent. Based on the experimental findings, some general conclusions about the dependence of the Soret coefficient on molecular properties are drawn.     

NMR shielding constants in hydrogen molecule isotopomers

We analyze the NMR shielding constants in three isotopomers of the hydrogen molecule: H₂, HD and D₂. The results obtained within the Born?COppenheimer approximation using the coupled-cluster singles-and-doubles model are very close to the previous theoretical values. In particular, the isotope shifts computed using significantly larger basis sets agree with the earlier literature results, confirming the disagreement of these calculations with the available experimental data. To examine the accuracy of the computed isotope shifts, we analyze in addition the relativistic corrections and estimate the role of the adiabatic and nonadiabatic effects. The relativistic corrections appear to be negligible; on the other hand, the changes in the shielding constants due to the adiabatic and nonadiabatic effects may account for the discrepancies between the computed and experimental isotope shifts.     

Separation of benzene and deuterated benzenes by reversed-phase and recycle liquid chromatography using monolithic capillary columns

An alternate pumping-recycle system utilizing a commercially available low dead-volume switching valve was developed for microcolumn LC. The recycle system had two separation columns, and the dead volume of the recycling lines was kept to a minimum by avoiding passage of the sample through the pump chamber, sample injector, and the normal path length of a conventional UV detector. The drawback of the high total back pressure caused by the second column that is placed after the detector was overcome by on-column detection, and this eliminated the need for a high pressure flow cell. The system was used for the separation of an authentic mixture of benzene, benzene-1,3,5-d3, and benzene-d6. Baseline separation was accomplished after six cycles and the calculated theoretical plate number for benzene was 230,000. It was observed that the theoretical plate number (N) increased linearly with increasing number of cycles, and the N per unit time increased with increasing inlet pressure. The separation conditions were optimized and the separation of benzene and benzene-d6 was accomplished within 75 min at 2.5 MPa inlet pressure.     

Theory of damped quantum rotation in nuclear magnetic resonance spectra. II. Numerical simulations for the benzene rotor

In Part I of this series of papers, the damped quantum rotation (DQR) theory, formulated originally for hindered threefold molecular rotors in solids, was generalized to the N-fold case. The stochastic dynamics of such objects, evidenced in NMR line shapes, was shown to be more complicated than in the standard model of classical jumps between the wells of the N-fold torsional potential. Actually, it comprises certain quantum rate (i.e., coherence-damping) processes subject to the requirements of the Pauli principle. The jump picture is recovered only when the quantum rates fit specific patterns. In this work, one of the ways of approaching such a classical limit is identified for the benzene rotor. This is inferred from a quantum mechanical model whose validity was earlier confirmed for a methyl group. Based on that model, theoretical calculations for the benzene ring dynamics in a clathrate crystal, 1-(9-anthryloxy)anthraquinone/benzene-d6, confronted with the pertinent literature data, point to possible deviations from the classical limit. However, the predicted DQR effects are too small to be observed in solid echo 2H NMR spectra of the C6D6 isotopomer. The chances of detecting the effects are improved when Carr-Purcell echo 1H spectra of a single crystal of the isotopomer including C6H6 as a guest are considered. The substantial differences in the sensitivity to the DQR effects of the spectra of protonated and deuterated benzene are concerned with different magnitudes of the intramolecular dipolar spin couplings. The dynamic isotope effect (C6D6 vs C6H6), which is small in this case, is only of secondary importance. Legitimacy of the use of the jump model in 2H NMR line shape studies of benzene-d6 is fully confirmed by the present considerations. However, the physical significance of the dynamic parameters extracted from such studies is shown from a new perspective.     

Calculation of the deuteron quadrupole relaxation rate in a mixture of water and dimethyl sulfoxide

An approach is presented that allows NMR relaxation rates to be determined for a complex mixture, and it is applied to a dimethyl sulfoxide/water solution. This approach is novel for such systems, having only been used for simple systems such as atomic liquids or atomic ions in liquids until now. It involves use of a predetermined, quantum mechanical, multidimensional property surface in a simulation. The results are used in conjunction with the simulated rotational correlation time to calculate the deuteron quadrupole coupling constant (DQCC), in an analogous approach to the one used by experimentalists, and to examine the surprising experimental findings for the composition dependence of the DQCC in the dimethyl sulfoxide/water mixture. Experiments have suggested that the DQCC for a mixture of 5% dimethyl sulfoxide in water is close to the DQCC of ice, whereas its value increases to a value close to the gas value with further dilution.(1) The results are further critically analyzed using combinations of different experimental and theoretical results from the literature.     

Relativistic spin-orbit coupling effects on secondary isotope shifts of (13)C nuclear shielding in CX(2) (X = O, S, Se, Te)

Rovibrational corrections, temperature dependence, and secondary isotope shifts of the (13)C nuclear shielding in CX(2) (X = O, S, Se, Te) are calculated taking into account the relativistic spin-orbit (SO) interaction. The SO effect is considered for the first time for the secondary isotope shifts. The nuclear shielding hypersurface in terms of nuclear displacements is calculated by using a density-functional theory method. Ab initio multiconfiguration self-consistent field calculations are done at the equilibrium geometry for comparison. (13)C NMR measurements are carried out for CS(2). The calculated results are compared with both present and earlier experimental data on CO(2), CS(2), and CSe(2). The heavy-atom SO effects on the rovibrational corrections of (13)C shielding are shown to be significant. For CSe(2) and CTe(2), reliable prediction of secondary isotope effects and their temperature dependence requires the inclusion of the SO corrections. In particular, earlier discrepancies of theory and experiment for CSe(2) are fully resolved by taking the SO interactions into account.     

Ultrafast orientational dynamics of nanoconfined benzene

Ultrafast optical Kerr effect spectroscopy has been used to study the orientational dynamics of benzene and benzene-d(6) confined in nanoporous sol-gel glass monoliths with a range of average pore sizes. All of the observed orientational diffusion of confined benzene is found to occur on a slower time scale than in the bulk, even in pores with diameters that are significantly larger than a benzene molecule. The orientational dynamics of benzene-d(6) are found to be inhibited to a lesser extent than those of benzene, which is attributed to the differences in wetting properties of the two liquids on silica. The decays are fit well by a sum of two exponentials, the faster of which depends on pore size. Similar results are found in pores that have been modified with trimethylsilyl groups, although the relaxation is faster than in unmodified pores. Comparison to Raman line width data for confined benzene-d(6) suggests that the liquid exhibits significant structuring at the pore walls, with the benzene molecules lying flat on the surfaces of unmodified pores.     

Dipole moment derivatives with respect to the internal coordinates of benzene in the liquid and gas phases

This paper presents a comparison of the dipole moment derivatives with respect to internal coordinates in the liquid and gas phases for benzene-h(6), benzene-d(6) and benzene-d(1). The literature values of the integrated intensities of the infrared active fundamentals of the three gaseous isotopomers are used to determine the dipole moment derivatives with respect to internal coordinates, using the methods described in the previous paper for the liquid phase. As was found for the liquid phase in the previous paper, there is uncertainty surrounding the intensities of the individual CH stretching fundamentals of benzene-d(1) due to intensity sharing with active combinations. The magnitudes of the dipole moment derivatives with respect to internal coordinates in the gas phase are partial differentialmicro/ partial differentials=0.50+/-0.03DA(-1), partial differentialmicro/ partial differentialt=0.28+/-0.03, partial differentialmicro/ partial differentialbeta=0.24+/-0.01, and partial differentialmicro/ partial differentialgamma=0.65+/-0.02DA(-1), where s, t, beta and gamma are the CH stretching, and CC stretching, the HCC bending and the HCCC torsion displacements, respectively. The experimental intensities are different for the three isotopomers in the liquid and gas phases, and the calculations show that these differences are mainly due to a difference between the CH stretch dipole moment derivatives in the two phases. This difference was related qualitatively to the intermolecular interaction of the H with the pi-cloud of the nearest neighbour creating a pseudo-hydrogen bond.     

Temperature-dependent optical Kerr effect spectroscopy of aromatic liquids

Ultrafast optical Kerr effect (OKE) spectroscopy has been used to study the temperature-dependent dynamics of five aromatic liquids: benzene, benzene-d(6), hexafluorobenzene, mesitylene, and 1,3,5-trifluorobenzene. The intermediate response time of all of the liquids was found to scale with the collective orientational correlation time, as has been observed for other simple liquids. The spectra of hexafluorobenzene, 1,3,5-trifluorobenzene, and mesitylene are qualitatively different from those of the other liquids and exhibit different behavior with temperature. These spectra allow us to assess the influence of different molecular parameters on the shape of the OKE spectrum. On the basis of these data, we propose a model that links the differences in the OKE spectra to corresponding differences in the local ordering of the liquids.     

Intrinsic information content of NMR dipolar couplings: a conformational investigation of 1,3-butadiene in a nematic phase.

The conformational equilibrium of 1,3-butadiene in a condensed fluid phase is investigated by liquid-crystal NMR spectroscopy. The full set of D(HH) and D(CH) dipolar couplings is determined from the analysis of the (1)H spectra of the three 1,3-butadiene most-abundant isotopomers (i.e. the all (12)C and the two single-labeled (13)C isotopomers) for a total of 21 independent dipolar couplings. A very good starting set of spectral parameters for the analysis of the (1)H spectrum is determined in a semiautomated way by the analysis of the (N-1) (specifically, N=6, the number of 1/2 spin nuclei in the spin system) quantum refocused (5QR), and not (5Q), spectra. As an alternative approach, a Monte Carlo (MC) numerical simulation, capable of predicting the solute ordering, is tested to simulate the 5QR spectrum. The set of D(ij) couplings is very good, proving that the MC method can represent a novel, valid alternative to the existing spectral simplification procedures. The experimentally determined dipolar-coupling data set is fully compatible with the 1,3-butadiene conformational distribution reported in the literature for isolated molecules, indicating the presence of about 99 % of s-trans conformer. With regards to the remaining 1 %, in spite of the direct and very strong dependence of the observables on the molecular structure, it was not possible to discriminate between the planar s-cis and s-gauche forms, both of which produce a very good fit of the dipolar couplings. Vibrational corrections, up to the anharmonic term, were applied; the calculated geometrical parameters are in good- although not exact-agreement with those reported in the literature from experimental and theoretical investigations. This result can be considered as supporting the methodology used for obtaining the structure and conformational distribution of a flexible molecule in a liquid phase.     

Dynamical consequences of symmetry breaking in benzene and difluorobenzene

The systems benzene/benzene-d(1) and o-/m-/p-difluorobenzene were studied in the dense gas phase with ultrafast transient absorption spectroscopy to investigate the effect of symmetry reduction through monodeuteration and constitutional isomerism on the timescales of intramolecular vibrational energy redistribution (IVR). In both systems IVR proceeds faster in the molecules of lower symmetry. In addition the dynamics were simulated in vibrational quantum number space using a simple model based on scaling state-to-state interactions by coupling order and the energy gap law. These simulations (semi-) quantitatively reproduce the experimental data for benzene and benzene-d(1) without incorporating further molecular symmetry restrictions. The relative impact of molecular symmetry and vibrational state space structure on IVR is discussed.     

Molecules with large-amplitude torsional motion partially oriented in a nematic liquid crystal: ethane and isotopomers

An NMR study on ethane and five isotopomers dissolved in the nematic liquid crystal Merck ZLI 1132 is performed. A consistent set of dipolar and quadrupolar couplings is obtained. The dipolar couplings are corrected for harmonic vibrational effects, while the contribution from the torsional motion is incorporated classically. The corrected dipolar couplings cannot be understood in terms of a reasonable molecular structure unless effects of the reorientation-vibration interaction are taken into account. Assuming that the reorientation-vibration contributions that are known for the methyl group in methyl fluoride are transferable to ethane, excellent agreement between observed and calculated dipolar couplings is obtained on the basis of the ethane gas-phase structure. The observed and calculated deuterium quadrupolar couplings show discrepancies supporting the notion that average electric field gradients are important in liquid-crystal solvents. An important consequence of the transferability of the reorientation-vibration correlation is that in other molecules with a methyl group the same procedure as for ethane can be followed. Inclusion of this effect generally removes the need to interpret changes in observed dipolar couplings in terms of elusive chemical effects.     

Comparison of the experimental, semi-experimental and ab initio equilibrium structures of acetylene: influence of relativisitic effects and of the diagonal Born-Oppenheimer corrections

The equilibrium structure of acetylene (also named ethyne) has been reinvestigated to resolve the small discrepancies noted between different determinations. The size of the system as well as the large amount of available experimental data provides the quite unique opportunity to check the magnitude and relevance of various contributions to equilibrium structure as well as to verify the accuracy of experimental results. With respect to pure theoretical investigation, quantum-chemical calculations at the coupled-cluster level have been employed together with extrapolation to the basis set limit, consideration of higher excitations in the cluster operator, inclusion of core correlation effects as well as relativistic and diagonal Born-Oppenheimer corrections. In particular, it is found that the extrapolation to the complete basis set limit, the inclusion of higher excitations in the electronic-correlation treatment and the relativistic corrections are of the same order of magnitude. It also appears that a basis set as large as a core-valence quintuple-zeta set is required for accurately accounting for the inner-shell correlation contribution. From a pure experimental point of view, the equilibrium structure has been determined using very accurate rotational constants recently obtained by a "global analysis" (that is to say that all non-negligible interactions are explicitly included in the Hamiltonian matrix) of rovibrational spectra. Finally, a semi-experimental equilibrium structure (where the equilibrium rotational constants are obtained from the experimental ground state rotational constants and computed rovibrational corrections) has been obtained from the available experimental ground-state rotational constants for ten isotopic species corrected for computed vibrational corrections. Such a determination led to the revision of the ground-state rotational constants of two isotopologues, thus showing that structural determination is a good method to identify errors in experimental rotational constants. The three structures are found in a very good agreement, and our recommended values are r(CC) = 120.2958(7) pm and r(CH) = 106.164(1) pm.     

The Role of the Ethynyl Substituent on the π–π Stacking Affinity of Benzene: A Theoretical Study

Herein, we report a high‐level theoretical study (SCS‐RI‐MP2(full)/aug‐cc‐pVTZ) examining the stacking affinity of 1,3,5‐triethynylbenzene. The stacking properties of this compound are compared to those of benzene and 1,3,5‐trifluorobenzene. The results indicate that the ethynyl substituent improves the stacking affinity of the arene, since the binding energies for the stacked ethynyl‐substituted arene dimers are higher than those of both benzene and the fluoro‐substituted arene. This interesting behaviour has been studied by examining the energetics, geometries and electron charge density features of the complexes. A query in the Cambridge Structural Database returned several X‐ray crystal structures containing π–π stacking interactions of 1,3,5‐triethynylaryls that strongly agree with the theoretical results.     

Electron distribution and molecular motion in crystalline benzene: an accurate experimental study combining CCD X-ray data on C6H6 with multitemperature neutron-diffraction results on C6D6

The electronic properties of the benzene molecule, for example its quadrupole moment and the electric field gradients (EFG's) at the H nuclei, are of fundamental importance in theoretical and experimental chemistry. With this in mind, single-crystal X-ray diffraction data on C(6)H(6) were collected with a charge-coupled device detector at T approximately 110 K. As accurate modelling of the thermal motion in the crystal was regarded as vital, especially for the hydrogen atoms, anisotropic-displacement parameters (ADP's) for the C and H atoms in C(6)H(6) were derived in a straightforward fashion from analysis of the temperature dependence of ADP's for the C and D atoms in C(6)D(6) at 15 K and 123 K obtained by neutron diffraction. Agreement between C-atom ADP's derived from thermal-motion analysis of neutron data and those obtained from multipole refinement by using the X-ray data is extraordinarily good; this gives confidence in the modelling of vibrational motion for the H atoms. The molecular quadrupole moment derived from the total charge density of the molecule in the crystal is (-29.7+/-2.4)x10(-40) C m(2), in excellent agreement with measurements made in the gas phase and in solution. The average deuterium nuclear quadrupole coupling constant (DQCC) derived from EFG tensors at H atoms is 182+/-17 kHz, also in excellent agreement with independent measurements. The strategy employed in this work may be of more general applicability for future accurate electron density studies.     

Some aspects of scaling factor calculations for quantum-mechanical molecular force fields

Stages of the scaling procedure for correcting molecular force fields obtained in rough quantum chemical calculations are discussed. The procedure includes selection of the number of scaling factors and their determination using experimental vibration frequencies of isotopomers and related molecules. It is stated that the difference in anharmonicity corrections for light molecules and their heavy analogs is to be taken into account along with possible errors in vibration frequency assignments of both isotopomers and more complex related molecules.     

Molecular parameters and mesomorphism of discotic polysubstituted benzene derivatives

The previously developed approach to prediction of columnar and nematic mesophases for compounds with disc-shaped molecules was applied to low-molecular-weight polysubstituted benzene derivatives. The molecular parameters were calculated for 39 new compounds: derivatives of 1,3,5-tri(4-hydroxybenzoyloxy)benzene, 1,3,5-tri(4-cyclohexylbenzoyloxy)benzene, 1,3,5-tri(4-hydroxyphenylcarboxy)benzene, 1,2,3-tri(4-cyclohexylbenzoyloxy)benzene, etc. The prediction results were checked by the synthesis of certain compounds from the test sample and examination of their mesomorphic behavior. The prediction of the mesomorphism is negative for only three of the 39 compounds considered. The majority of the molecules under consideration tend to form columnar supramolecular packings, and the capability to form a nematic phase is predicted for only eight compounds. For a number of structures, the capability and incapability to form a nematic phase are equally probable. Synthesis and examination of the mesomorphic behavior of seven repesentatives of the series considered showed good agreement with the prediction results. 1,3,5-Tri(4-heptyloxybenzoyloxy)benzene, 1,3,5-tri(4-cyclohexylbenzoyloxy)benzene, 1,3,5-tri(4-nonyloxyphenylcarboxy)-benzene, and 1,2,3-tri(4-cyclohexylbenzoyloxy)benzene pass into the liquid crystal state forming mainly columnar supramolecular packings. 1,3,5-Tri(4-octyloxybenzoyloxy)benzene and 1,3,5-tri(4-nonyloxybenzoyloxy)benzene melt at 55 and 48°C, respectively, without forming a mesophase, which is also well consistent with the prediction results.     

Influence of organics on the structure of water adsorbed on activated carbons

The influence of organics on the structure of water adsorbed on activated carbons was studied using adsorption of nitrogen, benzene, and water, and by (1)H NMR spectroscopy with freezing out of bulk water with the presence of benzene-d(6) or chloroform-d. It was found that interactions of water with the activated carbon surface depend on both structural characteristics (contributions of micro- and mesopores, pore size distributions) of adsorbents and chemical properties (changed by oxidation or reduction) of the adsorbents. Moreover, the interfacial behavior of water is affected by water-insoluble organics such as benzene and chloroform. Changes in the Gibbs free energy of water adsorbed on carbons exposed to air, water, chloroform-d, or benzene-d(6) are related to textural properties of adsorbents and the degree of their oxidation. Since chloroform-d and benzene-d(6) are strongly adsorbed on activated carbons and immiscible with water they replace a significant portion of adsorbed water in micropores, on the walls of mesopores, and in the transport pores of carbons causing changes in the Gibbs free energy and other characteristics of water.     

Additivity of ring distortions in halogen-substituted aromatics: a gas-phase electron diffraction and computational study

The gas-phase structures of 1,2,3-trifluorobenzene, 1,3,5-trifluorobenzene, 2,6-difluoropyridine, and 2,6-dichloropyridine have been determined using electron diffraction and computational methods. The accuracy afforded by modern experimental methods has allowed subtle trends to be identified in the geometrical parameters of the rings. Comparisons have also been made between experimental and theoretical structures. Although the effects of multiple fluorine substituents on a benzene ring are shown to be more or less additive, distortions caused by replacement of a ring carbon atom by nitrogen and by halogen substituents are not.