The millimetre-wave rotational spectrum of tertiary butyl cyanide

The rotational spectrum of tertiary butyl cyanide was studied in the frequency region up to 330 GHz at medium resolution using free-running, frequency-modulated backward wave oscillator sources. For the highest J transitions recorded, the K structure in the ground state was continuously observable up to well beyond K = 40. More than 100 ground-state transition frequencies have been measured, leading to accurate values for the rotational constant and quartic and sextic centrifugal distortion constants.     

The rotational spectrum and structure of the phosphine-hydrogen cyanide complex

The rotational spectrum of the weakly bound complex (PH₃, HCN) in its vibrational ground state has been observed by the technique of pulsed-nozzle, Fourier-transform microwave spectroscopy. The isotopic species (PH₃, HC¹⁴N), (PH₃, DC¹⁴N) and (PH₃, HC¹⁵N) exhibit spectra of the symmetric-top type from which accurate values of the spectroscopic constants B₀, D_J, D_JK and X_aa(¹⁴N) have been determined. For (PH₃, HC¹⁴N) the appropriate values are: B₀ = 1553.3709(1) MHz, D_J = 3.306(3) kHz, D_JK = 256.9(6) kHz and X_aa(¹⁴N) = ?4.3591(14) MHz. The geometry of the complex established from the spectroscopic constants is one of C_3v symmetry at equilibrium, with the HCN molecule lying along the C₃ axis of PH₃ and oriented so that it forms a hydrogen bond to the P atom. The effective distance from P to the C nucleus is r(P ? C) = 3.913 Å.     

Unusual structural isomerization of monopropiophenone thiocarbonohydrazone

Monopropiophenone thiocarbonohydrazone has been isolated in both linear and cyclic isomeric forms. Each form has been shown to isomerize and exist in equilibrium with the other in DMSO-d₆ solution by ¹H and ¹³C NMR spectroscopy. The kinetics of this transformation show attainment of equilibrium in approximately 6 h, with a linear to cyclic configuration ratio of 40:60.     

Infrared spectra and rotational isomerization of 1-bromo-3-methyl-2-butene

Infrared spectra obtained for the title compound, (CH₃)₂CCH. CH₂Br, have revealed the co-existence of two rotational isomeric forms in the fluid phases, the geometrical structures of which cannot be ascertained by use of vibrational spectroscopy alone. The energy ditterence between the two isomers was found to be 1.1 kcal mol⁻¹ (mean value) in the liquid state. Some of the i.r. fundamental bands were assigned using the characteristic absorption frequencies of localized group.     

Ultrafast structural and isomerization dynamics in the Rydberg-exited quadricyclane: norbornadiene system

The quadricyclane-norbornadiene system is an important model for the isomerization dynamics between highly strained molecules. In a breakthrough observation for a polyatomic molecular system of that complexity, we follow the photoionization from Rydberg states in the time-domain to derive a measure for the time-dependent structural dynamics and the time-evolving structural dispersion even while the molecule is crossing electronic surfaces. The photoexcitation to the 3s and 3p Rydberg states deposits significant amounts of energy into vibrational motions. We observe the formation and evolution of the vibrational wavepacket on the Rydberg surface and the internal conversion from the 3p Rydberg states to the 3s state. In that state, quadricyclane isomerizes to norbornadiene with a time constant of τ(2) = 136(45) fs. The lifetime of the 3p Rydberg state in quadricyclane is τ(1) = 320(31) and the lifetime of the 3s Rydberg state in norbornadiene is τ(3) = 394(32).     

High-dimensional mixed-valence copper cyanide complexes: Syntheses,structural characterizations and magnetism

Reactions of CuCl₂ with different CN complexes in presence of a neutral ancillary ligand lead to two novel mixed-valence Cu complexes [Cu^II(bpy)Cu^I(CN)₃]_n, 1 (bpy = 2,2′-bipyridine) and {[Cu^II(tn)₂][Cu^I₄(CN)₆]}_n2 (tn = 1,3-diaminopropane). For compound 1, the asymmetric unit involves two Cu ions Cu1 and Cu2 (Cu^I and Cu^II centres, respectively) which strongly differ in their environments. The Cu1 ion presents a CuC₄ pseudo-tetrahedral geometry, while the Cu2 ion presents a CuN₅ slightly distorted square-pyramidal geometry. The extended structure of 1 is generated by three cyano ligands which differ in their coordination modes. One CN group has a μ₃ coordination mode and bridges two Cu^I and one Cu^II ion, while the two other CN groups act as μ₂ bridges leading to a sophisticated 3-D structure. As for 1, the asymmetric unit of 2 involves three crystallographically different Cu ions (Cu1A and Cu1B, presumably Cu^I centres, and Cu2 presumably Cu^II centres). The Cu2 ion presents centrosymmetric CuN₄ coordination environments involving four nitrogen atoms from two bidentate tn ligands; while the Cu1A and Cu1B ions are three coordinated to cyano groups. The structure can be described as formed by 18-membered “[Cu^I(CN)]₆” planar metallocycles that are connected to their six neighbors to generate 2-D sheets; these sheets stack forming infinite hexagonal channels in which the [Cu(tn)₂]²⁺ units are located. Magnetic measurements show an unexpected weak ferromagnetic coupling (θ = 0.239(1) K) of the Cu^II ions through the long and “a priori diamagnetic” –NC–Cu^I–CN– bridges in compound 1 and an essentially paramagnetic behavior in compound 2.     

Photochemical E-Z isomerization of meta-terphenyl-protected phosphaalkenes and structural characterizations

A series of phosphaalkenes, E-ArP=C(H)Ar' (Ar = 2,6-Mes2C6H3, Ar' = Ph (1a); Ar = 2,6-Mes2C6H3, Ar' = p-C6H4Br (2a); Ar = 4-Br-2,6-Mes2C6H2, Ar' = Ph (3a); Ar = 4-Br-2,6-Mes2C6H2, Ar' = p-C6H4Br (4a)) have been prepared by phospha-Wittig reactions and characterized. Exposure of these materials either to room light over an extended period of time (days) or to UV light (hours) produced equilibrium mixtures of the E and Z isomers (1b-4b) as indicated by 1H and 31P NMR spectroscopy. The structures of compounds 4a and 4b were determined by single-crystal X-ray diffraction methods. Variable-temperature (1)H NMR studies of 4b indicate hindered rotation about the P-CAr bond, with DeltaH(double dagger) = 13.8 kcal/mol and DeltaS(double dagger) = 1.3 eu. The electronic structures of E- and Z-PhP=C(H)Ph have been examined using density functional theory.     

A remarkable shape-catalytic effect of confinement on the rotational isomerization of small hydrocarbons

As part of an effort to understand the effect of confinement by porous carbons on chemical reactions, we have carried out density functional theory calculations on the rotational isomerization of three four-membered hydrocarbons: n-butane, 1-butene, and 1,3-butadiene. Our results show that the interactions with the carbon walls cause a dramatic change on the potential energy surface for pore sizes comparable to the molecular dimensions. The porous material enhances or hinders reactions depending on how similar is the shape of the transition state to the shape of the confining material. The structure of the stable states and their equilibrium distributions are also drastically modified by confinement. Our results are consistent with a doubly exponential behavior of the reaction rates as a function of pore size, illustrating how the shape of a catalytic support can dramatically change the efficiency of a catalyst.     

Further study of cyclopropane structural isomerization behind reflected shock waves

The homogeneous thermal isomerization of cyclopropane to propene was studied in the presence of large excesses (99.6%–99.8%) of argon or helium diluent. Reaction temperatures ranged from 1038°?1208°K, and total gas pressures were varied from 533 to 5097 torr. The comparative-rate single-pulse shock-tube method was used, with the well characterized decomposition of cyclohexene serving as the internal standard reaction. Comparison of the measured rate constants for cyclopropane isomerization with k_∞ values extrapolated from “preferred?” lower-temperature rate constants suggests that collision efficiencies for helium and argon relative to cyclopropane, under the present conditions, are β_c ≈ 0.04 and 0.05, respectively. Although the uncertainties are rather large, these results do not support the suggestion that rapidly declining β_c values are largely responsible for the anomalously low rate constants for this reaction at T≥1130°K previously reported by other workers.     

Photoinduced rotational isomerization mechanism of 2-chlorobenzaldehyde in low-temperature rare-gas matrices by vibrational and electronic spectroscopies

Rotational isomerization of 2-chlorobenzaldehyde in low-temperature rare-gas matrices has been investigated by vibrational and electronic spectroscopies with aids of the density functional theory (DFT) and configuration interaction single (CIS) calculations. Infrared spectrum of the less stable O-cis isomer, produced from the more stable O-trans isomer upon UV irradiation, is measured with an FT-IR spectrophotometer. The enthalpy difference between the O-cis and O-trans isomers is estimated to be 9.7±0.2 kJ mol⁻¹ from the temperature dependence of the infrared band intensities. Analyses of the infrared and electronic absorption spectral changes after UV irradiation and the phosphorescence spectra measured at various excitation wavelengths suggest that the rotational isomerization occurs via the intersystem crossing from S₁ to T₁.     

Ultrafast structural dynamics of in-cage isomerization of diiodomethane in solution

Despite extensive studies on the isomer species formed by photodissociation of haloalkanes in solution, the molecular structure of the precursor of the isomer, which is often assumed to be a vibrationally hot isomer formed from the radical pair, and its in-cage isomerization mechanism remain elusive. Here, the structural dynamics of CH₂I₂ upon 267 nm photoexcitation in methanol were probed with femtosecond X-ray solution scattering at an X-ray free-electron laser. The determined molecular structure of the transiently formed species that converts to the CH₂I–I isomer has the I–I distance of 4.17 Å, which is longer than that of the isomer (3.15 Å) by more than 1.0 Å and the mean-squared displacement of 0.45 Ų, which is about 100 times larger than those of typical regular chemical bonds. These unusual structural characteristics are consistent with either a vibrationally hot form of the CH₂I–I isomer or the loosely-bound radical pair (CH₂I˙⋯I˙).

The structural dynamics of in-cage isomerization of CH₂I₂ and the unusual structure of the loosely-bound isomer precursor were unveiled with femtosecond X-ray liquidography (solution scattering).     

Facile syntheses, structural characterizations, and isomerization of disiloxane-1,3-diols

In the hydrolysis reaction of dichlorosilanes having an intramolecular coordinating atom, dcisiloxane-1,3-diols, [(OH){o-(CH₃)₂NCH₂-C₆H₄}RSi]₂O(R=CH₂CH (1), C₆H₅ (2), o-(CH₃)₂NCH₂C₆H₄ (3), Me (4)), were obtained in high yields. The results of the crystal structure analyses of meso-2, rac-2a, rac-2b and 3 are reported. They showed strong intramolecular hydrogen bondings between the hydroxy group and the nitrogen atom. We have also found that the diastereomeric isomerization of meso-2 to rac-2 in CDCl₃ solvent containing moisture occurred to result in the 55:45 equilibrium mixtures of the isomers and vice versa.     

Anomalously slow solvent structural relaxation accompanying high-energy rotational relaxation

Experimental and theoretical work on the relaxation of rapidly rotating solutes in liquids have pointed out a number of striking features. Even in rapidly relaxing solvents, the relaxation proceeds quite slowly, exhibiting a manifestly nonlinear response that depends explicitly on the initial rotational energy. In this paper, we show how the long-time behavior, in particular, stems from a strong coupling of solute orientation to local solvent geometry. This coupling creates a rotational friction that decreases sharply with rotational energy, allowing for the protracted survival of not only high-angular-momentum rotational states but the cavity-like low-friction solvent geometries. We show, further, that the slow dynamics is dynamically heterogeneous. The distribution of excited rotors is marked by a distinct population of slowly relaxing hot rotational states. This population can be traced directly to the small subset of liquid configurations that happen to have low rotational friction values at the instant at which the rapid rotation started, indicating an unusual failure of the normally chaotic environment of a liquid to randomize initial conditions.     

Kinematic effects associated with molecular frames in structural isomerization dynamics of clusters

Kinematic effects associated with movements of molecular frames, which specify instantaneous orientation of molecules, is investigated in structural isomerization dynamics of a triatomic cluster whose total angular momentum is zero. The principal-axis frame is employed to introduce the so-called principal-axis hyperspherical coordinates, with which the mechanism of structural isomerization dynamics of the cluster is systematically analyzed. A force called "democratic centrifugal force" is extracted from the associated kinematics. This force arises from an intrinsic non-Euclidean metric in the internal space and has an effect of distorting the triatomic cluster to a collapsed shape and of trapping the system around collinear transition states. The latter effect is particularly important in that the kinematics effectively makes a basin at the saddle (transition state) on the potential surface. Based on this framework, we study the effect of the gauge field associated with the Eckart frame in internal space, which has not been carefully examined in the conventional reaction rate theories. Numerical comparison between the dynamics with and without the gauge field has revealed that this field has an effect to suppress the rate of isomerization reaction to a considerable amount. Thus a theory neglecting this effect will significantly overestimate the rate of isomerization. We show the physical origin of this suppressing effect.     

Cyanide–Isocyanide isomerization in the structural isomers of cyanogen isocyanate

The cyanide-isocyanide isomerization has been studied with ab initio calculations in an STO -3G basis as applied to NCNCO, NCCNO, NCOCN, and NCONC, and the corresponding isocyanides. Geometry optimization has been performed on these cyanides, their isocyanides, and their hypothetical transition states. The energies of isomerization were calculated to be 42.2, 29.8, 44.6, and 41.4 kcal/mol, respectively, while the energy barriers were found as 84.3, 67.5, 107.9, and 106.8 kcal/mol. Overlap populations and atomic charges were employed to provide simple correlations of the results.     

Jet-cooled styrene: spectra and isomerization

The excitation and dispersed fluorescence spectra for the ùA′ ← X?¹A′ transition of styrene, cooled in a supersonic jet are reported. Assignment of the vibrational modes in the ground and excited electronic states are made and the excess energy dependence of the fluorescence spectra are analyzed in terms of their relevance to the dynamics of photoisomerization. We compare our results with those of stilbene and discuss the influence of IVR and mode mixing in the S₁ surface.     

Skeletal isomerization of butene in ferrierite: assessing the energetic and structural differences between carbenium and alkoxide based pathways

In this study, the results from a systematic analysis of two different mechanisms for the skeletal isomerization of cis-butene to isobutene in ferrierite (FER) are presented. One involves a conventional mechanism that proceeds via stable alkoxide intermediates and the other is one which proceeds via carbenium ions only. A 27T QM cluster model has been used in this study, which is described using the M06-2X DFT functional. It is found that the alkoxide intermediates formed over the course of the conventional pathway are considerably lower in energy than the carbenium ion formed over the course of the alternate pathway. However, the rate determining step in the latter pathway is predicted to be almost 10 kcal/mol lower in energy. The higher barrier for the latter process is due to the inherent stability of the alkoxide intermediates formed within FER. These results appear to suggest that while these intermediates are formed over the course of the reaction, the skeletal isomerization of linear butenes to form isobutene in FER may occur via a carbenium based mechanism. This proposal is consistent with experimental results that show alkoxide intermediates are experimentally observed species.