Rotational dynamics of methyl groups in m-xylene

Methyl group dynamics of m-xylene was investigated by using incoherent inelastic and quasi-elastic neutron scattering. Inelastic measurements were carried out at the high flux backscattering spectrometer HFBS at the National Institute of Standards, quasi-elastic measurements at the time-of-flight spectrometer NEAT at the Hahn-Meitner-Institute. Rotational potentials are derived which describe the tunnel splittings, first librational, and activation energies of the two inequivalent CH(3) groups. Indications for coupling of the methyl rotation to low-energy phonons have been found. The finite width of one tunneling transition at He temperature is described by direct methyl-methyl coupling. The combined results of the experiments and the calculations allow a unique assignment of rotor excitations to crystallographic sites.     

Ultraviolet photofragmentation spectroscopy of alkaline earth dication complexes with pyridine and 4-picoline (4-methyl pyridine)

A detailed experimental and theoretical study has been undertaken of the UV photofragmentation spectroscopy of the alkaline earth metal dications Mg(2+), Ca(2+), and Sr(2+) complexed with pyridine and 4-methyl pyridine (4-picoline). The ion complexes have been prepared using the pick-up technique and held in an ion trap where their internal temperature has been reduced to <150 K. Exposure of the trapped ions to tunable UV laser radiation leads to the appearance of photofragments with intensities that show significant variation as a function of wavelength. For all three metal dications, the resultant spectra show evidence of resolved features. Time-dependent density functional theory (TDDFT) has been used to identify possible electronic transitions that might be present in the [M(pyridine)(4)](2+) complexes (M = Mg, Ca, and Sr) within the wavelength range studied. These calculations show that the spectra are dominated by strong π* ← π and weaker π* ← n transitions localized on the pyridine ligands. The calculations correctly identify those regions of the experimental spectra where UV transitions begin to occur in the complexes and also the wavelengths at which absorption maxima are reached; however, more subtle features of the spectra are difficult to assign with confidence.     

On the microstructure of methyl acrylate-4-vinyl pyridine copolymers

Using ¹H-NMR spectra of radical copolymers of methyl acrylate (M₁) and 4-vinyl pyridine (M₂), the terminal model of copolymerization has been tested. Reactivity ratios (r₁ = 0.18 r₂ = 1.77) were found. The probability of “coisotactic” alternating addition (σ = 0.98) has been obtained as the best fit of experimental methoxy signal fractions and the theoretical curves.     

Penultimate effects in methyl methacrylate-4-vinyl pyridine copolymers

From the ¹H-NMR spectra of methyl methacrylate (M₁)-4-vinyl pyridine (M₂) radical copolymers with various monomer ratios, the reactivity ratios have been found using the penultimate model (r₁₁ = 1.51 r₂₁ = 0.10 r₂ = 0.24) and the co-isotactic alternating addition probability (σ = 0.5) as the best fit of the pentad distribution between the three parts of the methoxy signal.     

The relationship between crystal structure and methyl and t-butyl group dynamics in van der Waals organic solids

We report x-ray diffractometry in a single crystal of 2-t-butyl-4-methylphenol (TMP) and low-frequency solid state nuclear magnetic resonance (NMR) proton relaxometry in a polycrystalline sample of TMP. The x-ray data show TMP to have a monoclinic, P2(1)/c, structure with eight molecules per unit cell and two crystallographically inequivalent t-butyl group (C(CH(3))(3)) sites. The proton spin-lattice relaxation rates were measured between 90 and 310 K at NMR frequencies of 8.50, 22.5, and 53.0 MHz. The relaxometry data is fitted with two models characterizing the dynamics of the t-butyl groups and their constituent methyl groups, both of which are consistent with the determined x-ray structure. In addition to presenting results for TMP, we review previously reported x-ray diffractometry and low-frequency NMR relaxometry in two other van der Waals solids which have a simpler structure. In both cases, a unique model for the reorientational dynamics was found. Finally, we review a similar previously reported analysis in a van der Waals solid with a very complex structure in which case fitting the NMR relaxometry requires very many parameters and serves mainly as a flag for a careful x-ray diffraction study.     

Liquid crystal properties of novel diacetylenes having a pyridine end group

Asymmetrical diacetylenes with a pyridine ring bonded directly to one end of the diacetylene unit and an aryl system containing a flexible alkoxy chain at the opposite end, have been synthesized and show liquid crystalline behaviour. The mesophase behaviour is shown to be dependent on the length of the flexible alkoxy chain. Incorporation of one of these diacetylenes in a polymer 'guest-host' system at a 15 wt% concentration gave a high X⁽²⁾ non-linear optical susceptibility of 4.27 × 10^-9e.s.u.     

Computational study of methyl group dynamics in the hydroquinone clathrate of acetonitrile

We report molecular dynamics simulations of the acetonitrile clathrate of hydroquinone, with a focus on the dynamics of acetonitrile methyl groups. There are three inequivalent acetonitrile molecules in the unit cell, one with its dipole parallel to the c-axis, and the other two antiparallel. Although these three guest molecules have previously been found to exhibit two slightly different frequencies of rotation over a wide range of temperatures, the frequencies could not be assigned to specific methyl groups. Perhaps counterintuitively, our simulations suggest that the molecule with the lower frequency is one of the two molecules oriented the same way, the different dynamical behaviour being due to subtle differences in the environments of the molecules.     

Rotational dynamics of a discotic liquid crystal studied by deuteron spin relaxation

We report on measurements of deuteron quadrupolar splitting and spin-lattice relaxation times T_1Q and T_1Z in the columnar phase of a ring-deuteriated hexaoctyloxyrufigallol at 46 MHz as a function of temperature. To describe small-step rotations of these molecules within each column in the columnar phase, a space-fixed frame is used to diagonalize the molecular diffusion tensor. The principal diffusion constants in this so called 'anisotropic viscosity' model D_alpha and D_beta are for rotations of a molecule around and perpendicular to the columnar axis, respectively. A global target analysis of the spectral densities at seven temperatures in a minimization procedure was carried out. We found that D_alpha > D_beta, which is consistent with the picture that the motion towards or away from the local director tends to disrupt the packing of molecules within the column.     

Reactions of 2, 5-dimethyl-4-triphenylsilylpyridine at the α-methyl group

5-Methyl-4-triphenylsilylpyridine-2-carboxylic acid was synthesized, esterified, and decarboxylated. Several transformations of trans-5-methyl-4-triphenylsilyl-2-styrylpyridine were examined.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 957–961, July, 1972.     

Flammability studies of 3-methyl pyridine/water system

In industrial processes, information on the safety property of chemicals is essentially crucial for safe handling during unit operations. Ensuring the safe use of combustible or flammable substances in processes is unlikely without detailed investigations of their flammability characteristics and related hazards. We studied 3-methyl pyridine (3-picoline), e.g., flammability limits (LFL/UFL), maximum explosion pressure (P _max), maximum explosion pressure rise (dP/dt)_max, minimum oxygen concentration (MOC), vapor deflagration index (K _g), and characterized the influence of inert steam (H₂O) on critical parameters for 3-picoline/water mixtures at 270°C, 1 atm, various oxygen concentrations, and vapor mixing ratios (100/0, 30/70, 10/90 and 5/95 vol.%) with a 20-L-Apparatus in simulated conditions, respectively. The results showed that the flammability characteristics of 3-picoline_(aq) all increased with the oxygen concentration. However, as the composition of inert steam increased, the flammability parameters and the degree of fire and explosion hazards were significantly reduced, instead. This study elucidated the flammability properties of 3-picoline mixed with inert steam. The conclusions could be applied to proactively prevent the relevant processes from incurring fire and explosion accidents.     

Atmospheric ozonolysis study of methyl acrylate and methyl 3-methyl acrylate

Detailed reaction mechanisms of methyl acrylate (MA) and methyl 3-methyl acrylate (M3MA) with ozone have been investigated using quantum chemistry calculations based on the CCSD(T)/6-31G(d)+CF//B3LYP/6-31+G(d,p) level of theory. Possible reaction channels and products have been presented and discussed. The temperature-dependent and pressure-independent overall and site-specific rate constants are calculated by employing multichannel RRKM theory. The obtained overall rate constants (in cm³ molecule^?1 s^?1) based on the CCSD(T)+CF energies can be described as: $ k_{{({\text{MA}} + {\text{O}}_{ 3} )}} = 3. 7 4 \times 10^{ - 1 3} { \exp }\,( - 3 7 4 6. 1 2/T) $ and $ k_{{({\text{M3MA}} + {\text{O}}_{ 3} )}} = 5. 1 2 \times 10^{ - 1 3} { \exp }( - 3 3 5 4. 8 7/T) $ at 200–400 K and 760 Torr. Under atmospheric conditions, the dominant products of ozonolysis of MA and M3MA are methyl glyoxylate and formaldehyde, methyl glyoxylate and acetaldehyde, respectively. The results of theoretical study are in good agreement with the available experimental measurements. Researches on branching ratios and atmospheric lifetimes also have been obtained as complement to the experimental results.     

Rotational isomerism in 1,3-dibromopropane and 1-bromo-3-methyl pentane

Gas, liquid and solution phase i.r. measurements are reported on 1,3-dibromopropane and 1-bromo-3-methyl-pentane. Enthalpy differences and potential barriers to internal rotation were determined from respectively the temperature dependence of the intensity of the carbon-bromine stretching frequencies and the positions of the torsional modes. The experimental data is compared with the results of theoretical calculation. In both cases reasonable agreement was observed between experiment and theory. The effects of dipolar interactions are discussed.     

Photodissociation dynamics of pyridine

Photodissociation of pyridine, 2,6-d2-pyridine, and d5-pyridine at 193 and 248 nm was investigated separately using multimass ion imaging techniques. Six dissociation channels were observed at 193 nm, including C5NH5 --> C5NH4 + H (10%) and five ring opening dissociation channels, C5NH5 --> C4H4 + HCN, C5NH5 --> C3H3 + C2NH2, C5NH5 --> C2H4 +C3NH, C5NH5 --> C4NH2 + CH3 (14%), and C5NH5 --> C2H2 + C3NH3. Extensive H and D atom exchanges of 2,6-d2-pyridine prior to dissociation were observed. Photofragment translational energy distributions and dissociation rates indicate that dissociation occurs in the ground electronic state after internal conversion. The dissociation rate of pyridine excited by 248-nm photons was too slow to be measured, and the upper limit of the dissociation rate was estimated to be 2x10(3) s(-1). Comparisons with potential energies obtained from ab initio calculations and dissociation rates obtained from the Rice-Ramsperger-Kassel-Marcus theory have been made.     

Molecular dynamics simulation of the interaction between methyl benzotriazole and Cu2O crystal

In circulating water system , methyl benzotriazole (TTA) is one of the common corrosion inhibitors for copper. But the inhibition mechanisms have not been clearly understood so far. In different number of water molecules, the interaction between TTA and Cu₂O (copper surface) was investigated with molecular dynamics (MD) method. The results showed that the MD simulation result with water was more consistent with the experiment results. In different number of water molecules, the sequence of the interaction energies between TTA and Cu₂O (001) was ? E ₁ (150H₂O) > ? E ₁(200H₂O) > ? E ₁(100H₂O) > ? E ₁(50H₂O) > ? E ₁(0H₂O). The number of water molecules had an important influence on the interaction between corrosion inhibitors and Cu₂O crystal. From non‐bond energy and pair correlation functions, the interaction energies of the model system were mainly contributed by the non‐bond interaction. Strong adsorption could be raised by the Coulomb interaction between the negatively charged functional groups in TTA and the positive copper ions in the Cu₂O (001) face, and further interaction between aggressive media and copper could be restricted. So, copper corrosion could be avoided. Chemical bonds and non‐bond interactions were formed between TTA and Cu₂O (001) in different number of water molecules. Water molecules could not be ignored during the MD simulation, too. The results obtained here may provide theoretical supports for developing new corrosion inhibitors.     

Rotational isomerism in 3-methyl, 3-butenenitrile(methallyl cyanide)

Infrared spectra were obtained for the title compound, CH₂C (CH₃) CH₂ CN, in the liquid and solid states. The rotational isomerism of the molecule was investigated on the basis of solvents and temperature dependence of the absorption spectra. The results revealed the coexistence of three conformational isomers in the liquid state. The energy difference between two of them, the more and the less polar forms as inferred from the solvents variation study, was estimated to be 1.35 kcal/mol (mean value). A normal coordinate analysis assuming valence force field constants and structural parameters was carried out and the calculated frequencies were used as a guide in approximately assigning the observed fundamentals to their normal modes.     

Ground-state repopulation dynamics in pyridine vapor

The new experimental method of vibronically selective ground-state recovery probing has been used to study relaxation processes that follow S₁ ←     

Origin of threefold symmetric torsional potential of methyl group in 4-methylstyrene

To understand the effect of the para position vinyl group substitution in toluene on methyl torsion, we investigated 4-methylstyrene, a benchmark molecule with an extended pi conjugation. The assignment for a 33 cm(-1) band in the excitation spectrum to the 3a(2) torsional transition, in addition to the assignments suggested previously for the other bands in the excitation spectrum, leads to the model potentials for the ground as well as excited states with V(3) (")=19.6 cm(-1), V(6) (")=-16.4 cm(-1) and V(3) (')=25.6 cm(-1), V(6) (')=-30.1 cm(-1), respectively. These potentials reveal that both in ground and excited states, the methyl group conformations are staggered with a 60 degrees phase shift between them. MP2 ab initio calculations support the ground state conformations determined from experiments, whereas Hartree-Fock calculations fail to do so. The origin of the modified ground state potential has been investigated by partitioning the barrier energy using the natural bond orbital (NBO) theoretical framework. The NBO analysis shows that the local delocalization (bond-antibond hyperconjugation) interactions of the methyl group with the parent molecule is sixfold symmetric. The threefold symmetric potential, on the other hand, stems from the interaction of the vinyl group and the adjacent ring pi bond. The threefold symmetric structural energy arising predominantly from the pi electron contribution is the barrier forming term that overwhelms the antibarrier contribution of the delocalization energy. The observed 60 degrees phase shift of the excited state potential is attributed to the pi(*)-sigma(*) hyperconjugation between out of plane hydrogens of the methyl group and the benzene ring.     

Selective substitutions in the C10-methyl group in erythromycin derivatives

A method for chemical modifications of the relatively unreactive C10-methyl group in the erythromycin macrolactone ring has been developed. Erythralosamine was protected as an N-oxide in the N,N-dimethylamino group and reacted with NBS in acetic acid to provide two regioisomeric allylic bromides. The same amine was formed from both isomers on nucleophilic substitution. Both regioisomeric bromides in cross-coupling reactions under Stille conditions provided the same product from substitution in the 10-methyl group via a common π-allylic palladium complex. Under Negishi conditions with trimethylalane, the Pd-catalysed cross-coupling provided the 10-ethyl homologue. X-ray analyses were used to confirm the structure of erythralosamine, and to determine the structures of the allylic bromides from erythralosamine N-oxide.     

The impact of solvent polarity on intramolecular proton and electron transfer in 2-alkylamino-4-nitro-5-methyl pyridine N-oxides

The crystal structure of 2-butylamino-4-nitro-5-methyl pyridine N-oxide (2B5M) and solution studies of both 2B5M and 2-methylamino-4-nitro-5-methyl pyridine (2M5M) N-oxide are presented. Steady-state absorption and emission measurements were employed for both molecules while a picosecond fluorescence up-conversion technique was used to follow the dynamic behavior of the 2M5M system. The experimental methods were complemented by DFT and TD DFT B3LYP/6-31G(d,p) calculations involving ground and excited-state optimization which in the case of the smaller 2M5M molecule were extended to the CAM-B3LYP/6-31G(d,p) method. The solvent effect is incorporated by applying the polarizable continuum (PCM) model. The data reveal that the 2B5M molecule crystallizes in the monoclinic space group P2(1)/c and its crystal lattice is composed of monomers with intramolecular N-H···O [2.572(3) ?] hydrogen bonds, connected into a polymer network by weak intermolecular C-H…O [3.2-3.4 ?]-type interactions. Quantum-chemical calculations show that the aminoalkyl substitutent in aminoalkyl-pyridine N-oxides is a specific determinant of the CT nature of the lowest-lying excited electronic ππ* state, distinguishing them from other nitroaromatic compounds. The results of both picosecond fluorescence up-conversion experiments in different solvents and quantum-chemical calculations suggest that in nonpolar media the ESIPT process in 2M5M is favored, while in polar acetonitrile, the N* → PT* transition demands barrier-crossing and thus unfavorable thermodynamic conditions do not allow the ESIPT to occur. The signals of picosecond fluorescence up-conversion of 2M5M are solvent- and emission-wavelength dependent. The three time components found in a weakly polar isooctane-dioxane mixture have been attributed to solvation dynamics (～500 fs), and to relaxation of N* and PT* forms while in acetonitrile, a very rapid fluorescence decay with a time constant (2.3-4.0 ps) indicative of the presence of the normal (N*) form was observed. Much shorter fluorescence lifetimes in alcohols (a few picoseconds) and in D(2)O (less than 200 fs) than in aprotic solvents suggest that in protic media, the solvent molecules participate in the ESIPT, bridging between the methylamine group and the N-oxide group of 2M5M.