The dynamics of the CH3CN molecule in the isotropic phase and in a nematic solution

Abstract

NMR lineshape studies of acelonitrile in the isotropic and the liquid-crystalline nematic phase of PCH have been performed. The scalar relaxation of the second kind due to the presence of the ¹⁴N quadrupolar nucleus has been confirmed as the most important relaxation mechanism for this molecule in both the isotropic and the anisotropic phase. It has been found largely responsible for the selective broadening on ¹³C and ¹H transitions. A minor contribution arising from intramolecular dipolar relaxation mechanism has also been investigated. Linewidth analysis of the NMR spectra allowed us to determine the quadrupolar relaxation time T _IN of the ¹⁴N nucleus. This is connected to the correlation time for rotational diffusion perpendicular to the molecular symmetry axis. A possible explanation of a residual selective broadeining which effects the ¹³C and ¹H NMR transitions and is not taken into account by this mechanism, is also given.     

The formation of the stable radicals .CH2CN, CH3.CHCN and .CH2CH2CN from the anions -CH2CN, CH3-CHCN and -CH2CH2CN in the gas phase. A joint experimental and theoretical study

Franck-Condon one-electron oxidation of the stable anions -CH2CN, CH3-CHCN and -CH2CH2CN (in the collision cell of a reverse-sector mass spectrometer) produce the radicals .CH2CN, CH3.CHCN and .CH2CH2CN, which neither rearrange nor decompose during the microsecond duration of the neutralisation-reionisation experiment. Acetonitrile (CH3CN) and propionitrile (CH3CH2CN) are known interstellar molecules and radical abstraction of these could produce energised .CH2CN and CH3.CHCN, which might react with NH2. (a known interstellar radical) on interstellar dust or ice surfaces to form NH2CH2CN and NH2CH(CH3)CN, precursors of the amino acids glycine and alanine.     

On the twist-bend nematic phase formed directly from the isotropic phase

The intriguing twist-bend nematic (N_TB) phase is formed, primarily, by liquid crystal dimers having odd spacers. Typically, the phase is preceded by a nematic (N) phase via a weak first-order transition. Our aim is to obtain dimers where the N_TB phase is formed directly from the isotropic (I) phase via a strong first-order phase transition. The analogy between such behaviour and that of the smectic A (SmA)–N–I sequence suggests that this new dimer will require a short spacer. This expectation is consistent with the prediction of a molecular field theory, since the decrease in the spacer length results in an increase in the molecular curvature. A vector of odd dimers based on benzoyloxybenzylidene mesogenic groups with terminal ethoxy groups has been synthesised with spacers composed of odd numbers of methylene groups. Spacers having 5, 7, 9 and 11 methylene groups are found to possess the conventional phase sequence N_TB–N–I; surprisingly, for the propane spacer, the N_TB phase is formed directly from the I phase. The properties of these dimers have been studied with care to ensure that the identification of the N_TB phase is reliable.     

Diffusion of brownian particles in the isotropic phase of a nematic liquid crystal

Abstract

We report a light scattering study of the translational diffusion of a suspension of silica spheres in the liquid crystal, 4-n-pentyl-4'-cyanobiphenyl. We observe a small but significant increase of the effective hydrodynamic radius of the colloidal particles as the transition to the nematic phase is approached. This effect can be understood in terms of orientational pre-wetting of the silica spheres.     

Diffusion of brownian particles in the isotropic phase of a nematic liquid crystal

We report a light scattering study of the translational diffusion of a suspension of silica spheres in the liquid crystal, 4-n-pentyl-4'-cyanobiphenyl. We observe a small but significant increase of the effective hydrodynamic radius of the colloidal particles as the transition to the nematic phase is approached. This effect can be understood in terms of orientational pre-wetting of the silica spheres.     

On the observation of flow-induced order in the isotropic phase of a nematic discogen

The phenomenon of flow-induced order observed in the isotropic phase of nematic discotic materials is studied and the orientation of the induced director explained by the existence of the molecules in short columns. The occurrence of this order in both the isotropic and nematic phases explains the low transition enthalpy, agreeing with the inferences made in the literature.     

Influence of non-mesogenic impurities on a nematic to isotropic phase transition

Non-mesomorphic solutes depress the normal nematic-isotropic transition temperature in liquid crystals. When non-mesomorphic solutes are added to a nematic liquid crystal, the nematic-isotropic transition temperature is depressed and a two phase region is formed due to the presence of impurities of the solutes. The present paper explains the formation of this two phase region by the Landau-de Gennes phenomenological theory, which agrees fairly well with the experimental observations. We also note that this two phase region indicates the tricritical behaviour of the nematic-isotropic phase transition and the phase diagram near the tricritical point is also obtained.     

Dielectric studies and critical behaviour in the vicinity of nematic–isotropic phase transition of a smectogenic binary mixture showing induced nematic phase

The phase diagram of a binary mixture composed of compounds, one having NCS terminal group (4DBT, showing smectic A₁ phase) and the other with CN terminal group (11OCB, showing smectic A_d phase), exhibiting induced nematic phase in a certain concentration range (0.100 < x_4DBT < 0.951) is reported here. Results of the static dielectric parameters measurement on this binary system within the entire mesomorphic range are presented. Evidence of strong pretransitional behaviour near the nematic–isotropic (N–I) phase transition, indicating the influence of tricritical behaviour, is observed. Precise determination of discontinuity (ΔT) and the critical exponent (α) of N–I phase transition have been carried out. Moreover, the order parameter critical exponent β is correctly predicted by the tricritical hypothesis through the dielectric anisotropy data for all the investigated mixtures.     

An investigation of the dynamics of CH3I in a nematic mesophase by NMR linewidth analysis

Relaxation mechanism acting on CH₃I dissolved in a nematic mesophase were studied by linewidth analysis of ¹H and ¹³C NMR spectra. Intramolecular and intermolecular dipolar interaction fluctuations were found to be the most important relaxation phenomena responsible for the line broadening.     

Subdiffusive dynamics of a liquid crystal in the isotropic phase

The isotropic phase dynamics of a system of 4-n-hexyl-4'-cyano-biphenyl (6CB) molecules has been studied by molecular dynamics computer simulations. We have explored the range of 275-330 K keeping the system isotropic, although supercooled under its nematic transition temperature. The weak rototranslational coupling allowed us to separately evaluate translational (TDOF) and orientational degrees of freedom (ODOF). Evidences of subdiffusive dynamics, more apparent at the lowest temperatures, are found in translational and orientational dynamics. Mean square displacement as well as self-intermediate center of mass and rotational scattering functions show a plateau, also visible in the orientational correlation function. According to the mode coupling theory (MCT), this plateau is the signature of the beta-relaxation regime. Three-time intermediate scattering functions reveal that the plateau is related to a homogeneous dynamics, more extended in time for the orientational degrees of freedom (up to 1 ns). The time-temperature superposition principle and the factorization property predicted by the idealized version of MCT hold, again for both kinds of dynamics. The temperature dependence of diffusion coefficient and orientational relaxation time is well described by a power law. Critical temperatures Tc are 244+/-6 and 258+/-6 K, respectively, the latter is some 10 K below the corresponding experimental values. The different values of Tc we obtained indicate that ODOF freezes earlier than TDOF. This appears due to the strongly anisotropic environment that surrounds a 6CB molecule, even in the isotropic phase. The lifetime of these "cages," estimated by time dependent conditional probability functions, is strongly temperature dependent, ranging from some hundreds of picoseconds at 320 K to a few nanoseconds at 275 K.     

Vibrational relaxation of CH3I in the gas phase and in solution

Transient electronic absorption measurements reveal the vibrational relaxation dynamics of CH(3)I following excitation of the C-H stretch overtone in the gas phase and in liquid solutions. The isolated molecule relaxes through two stages of intramolecular vibrational relaxation (IVR), a fast component that occurs in a few picoseconds and a slow component that takes place in about 400 ps. In contrast, a single 5-7 ps component of IVR precedes intermolecular energy transfer (IET) to the solvent, which dissipates energy from the molecule in 50 ps, 44 ps, and 16 ps for 1 M solutions of CH(3)I in CCl(4), CDCl(3), and (CD(3))(2)CO, respectively. The vibrational state structure suggests a model for the relaxation dynamics in which a fast component of IVR populates the states that are most strongly coupled to the initially excited C-H stretch overtone, regardless of the environment, and the remaining, weakly coupled states result in a secondary relaxation only in the absence of IET.     

Study of the interaction between aniline and CH3CN, CH3Cl and CH3F

A computational study of dimers formed by aniline and one or two CH₃X molecules, X being CN, Cl or F, was carried out to elucidate the main characteristics of the interacting systems. Two different structures were found for each of the dimers, depending on the relative location of the CH₃X molecule with respect to the NH₂ hydrogen atoms. The most stable complex is formed with acetonitrile, with a complexation energy amounting to ?27.0?kJ/mol. Methyl chloride and methyl fluoride form complexes with complexation energies amounting to ?18.1 and ?17.5?kJ/mol, respectively, though the structural arrangement is quite different for both structures. In most complexes, the leading contribution to the stabilization of the complex is dispersion, though the electrostatic contribution is almost as important. Three different minima were obtained for clusters containing two CH₃X molecules depending on the side they occupy with respect to the phenyl ring. The complexation energies for these structures amount to ?58.5, ?38.6 and ?36.3?kJ/mol for acetonitrile, methyl chloride and methyl fluoride, respectively.     

Orientational dynamics in the isotropic phase of a calamitic liquid-crystal model

We report a molecular dynamics simulation study on the isotropic phase of an idealized calamitic liquid crystal model with a length-to-width ratio of approximately 5-6. The study focuses on the characterization of single-particle and collective orientational dynamics on approaching the phase transition to the nematic phase. Recent experimental and simulation works have suggested that a power law behavior exists at relatively short times in the decay of the time derivative of the orientational correlation functions. Qualitatively, our simulation data are consistent with these findings. Both single-particle and collective time correlation function derivatives possess, in their respective log-log plots, a linear region at very short times, whose slope is essentially independent from the thermodynamic state. Nevertheless, the single-particle orientational correlation functions are better described by a function which is the sum of a fast exponential, an intermediate stretched-exponential and a slow exponential, while the collective orientational correlation functions are satisfactorily described by a sum of two exponentials, at higher density, or by just one exponential, at lower density.     

State-resolved dynamics of the CN(B2Sigma+) and CH(A2Delta) excited products resulting from the VUV photodissociation of CH3CN

Fourier transform visible spectroscopy, in conjunction with VUV photons produced by a synchrotron, is employed to investigate the photodissociation of CH3CN. Emission is observed from both the CN(B2Sigma+-X2Sigma+) and CH(A2Delta-X2Pi) transitions; only the former is observed in spectra recorded at 10.2 and 11.5 eV, whereas both are detected in the 16 eV spectrum. The rotational and vibrational temperatures of both the CN(B2Sigma+) and CH(A2Delta) radical products are derived using a combination of spectral simulations and Boltzmann plots. The CN(B2Sigma+) fragment displays a bimodal rotational distribution in all cases. Trot(CN(B2Sigma+)) ranges from 375 to 600 K at lower K' and from 1840 to 7700 K at higher K' depending on the photon energy used. Surprisal analyses indicate clear bimodal rotational distributions, suggesting CN(B2Sigma+) is formed via either linear or bent transition states, respectively, depending on the extent of rotational excitation in this fragment. CH(A2Delta) has a single rotational distribution when produced at 16 eV, which results in Trot(CH(A2Delta))=4895+/-140 K in v'=0 and 2590+/-110 K in v'=1. From thermodynamic calculations, it is evident that CH(A2Delta) is produced along with CN(X2Sigma+)+H2. These products can be formed by a two step mechanism (via excited CH3* and ground state CN(X2Sigma+)) or a process similar to the "roaming" atom mechanism; the data obtained here are insufficient to definitively conclude whether either pathway occurs. A comparison of the CH(A2Delta) and CN(B2Sigma+) rotational distributions produced by 16 eV photons allows the ratio between the two excited fragments at this energy to be determined. An expression that considers the rovibrational populations of both band systems results in a CH(A2Delta):CN(B2Sigma+) ratio of (1.2+/-0.1):1 at 16 eV, thereby indicating that production of CH(A2Delta) is significant at 16 eV.     

Chiral symmetry-breaking dynamics in the phase transformation of nematic droplets

Dynamic simulations of the isotropic–nematic phase transformation of liquid crystal droplets with homeotropic surface anchoring are found to predict chiral symmetry-breaking dynamics. These observations occur when using material parameters for pentyl-cyanobiphenyl (5CB) but not with the single elastic constant approximation for this material, which is frequently used in simulations. The twisting dynamic process occurs during the relaxation of the domain from an unstable radial texture to a stable uniform texture and involves simultaneous defect loop motion and twisting of the bulk nematic texture.     

Pretransitional behavior in the isotropic phase of a nematic liquid crystal with the transverse permanent dipole moment

The results presented give the evidence for the quasicritical, pretransitional behavior of dielectric properties in the isotropic phase of a rodlike nematic liquid crystal with the transverse permanent dipole moment. Studies were conducted in 2-cyano-4-pentylbiphenyl 4-(trans-4-pentylcyclohexyl) benzoate, focusing on the static-and ionic-dominated low-frequency (LF) regions. For the static dielectric permittivity [epsilon(')(T)] the application of the derivative analysis revealed the pretransitional anomaly associated with the specific heat exponent alpha approximately 0.5. For the LF domain the contribution to epsilon(')(T) from residual ionic impurities follows a linear temperature dependence on approaching the isotropic-nematic (I-N) transition. This dependence and pretransitional anomalies of electric conductivity and dielectric modulus can be associated with the influence of prenematic fluctuations. "Linear" dielectric studies were supported by the static nonlinear dielectric effect measurements, which delivered reliable estimations of the temperature of the hypothetical continuous phase transition T(*) and the discontinuity of the I-N transition DeltaT approximately 1.7 K.     

The kinetics of nematic phase size growth during the phase transition in an isotropic melt of liquid crystalline azomethine dimer

Polarizing microscopy was used to study the kinetics of formation and droplet size growth of the ordered (nematic) phase during the phase separation of an azomethine dimer melt at various rates of cooling. The statistical droplet size distribution of the nematic phase during phase separation was described by a model derived in terms of the thermodynamics of irreversible processes. Two kinetic phase separation stages were observed and described by the universal scaling function.     

Rheology of a thermotropic polyester in the nematic and isotropic states

The rheological properties of a thermotropic polyester were determined in the nematic and isotropic states. In the isotropic state, the viscosity is almost constant and the polymer is only slightly elastic. The nematic phase has a lower viscosity than the isotropic, except at low frequencies or shear rates, where the viscosity increases as though the polymer had a yield stress. There is a marked dependence of the rheology on shear history. The effects of shearing can be erased by returning the material first to the isotropic state and then back to the nematic state. The results are discussed with reference to analogous observations in small-molecule liquid crystals and in thermotropic aromatic co-polyesters.     

Steric effects in electron transfer from potassium to pi-bonded oriented molecules CH3CN, CH3NC, and CCl3CN

Electron transfer from K atoms to oriented CH3CN, CH3NC, and CCl3CN is studied in crossed beams at energies near the threshold for forming an ion pair. For the methyl compounds, the dominant ions are K+ and CN-; the steric asymmetry is very small and energy-independent, characteristic of sideways attack with the electron apparently entering the pi*CN antibonding orbital. Migration of the electron to the sigma*CC orbital to break the C-C bond is greatly facilitated by interaction with the atomic donor. CH2CN- is formed in collisions preferring CH3-end attack, and the steric asymmetry becomes very large near threshold. CCl3CN mostly forms Cl- in collisions slightly favoring the CCl3 end with a small energy dependence with the electron apparently entering the sigma* LUMO. CN- is formed in much smaller yield with a slight preference for the CN end. The parent negative ion CCl3CN- is observed, and a lower limit for its electron affinity is estimated to be 0.3 eV. Fragment ions CCl2CN- and CClCN- are also observed with upper limits for the quantity bond dissociation energy - electron affinity (BDE - EA) estimated to be 0.6 and 1.0 eV, respectively.     

Kinetics of the nematic ordered phase growth during a temperature quench of an isotropic siloxane-azomethine polymer

Kinetics of the nucleus growth during a deep temperature quench across the isotropic to nematic phase transition was experimentally investigated for a siloxane-azomethine polyether at cooling rates of 10 and 20°C min^-1. Nematic droplets revealed in the optical images during the phase separation were treated statistically and the resulting statistical size distributions were described using the model of reversible aggregation. Analysis of the time-dependent distribution parameters allowed two processes involved in liquid crystal phase ordering to be identified: nucleus growth and nucleus coarsening. Both regimes are quantitatively described using the universal growth law.