Mahesh B. Dawadi

Communication: Conical intersections between vibrationally adiabatic surfaces in methanol

A set of seven conical intersections (CIs) in methanol between vibrationally adiabatic surfaces is reported. The intersecting surfaces represent the energies of the two asymmetric CH stretch vibrations regarded as adiabatic functions of the torsion and COH bend angles. The ab initio data are well described by an extended Zwanziger and Grant (E ⊗ e) model [J. W. Zwanziger and E. R. Grant, J. Chem. Phys. 87, 2954 (1987)] that might also be regarded as an extension of the XHL model [L.-H. Xu, J. T. Hougen, and R. M. Lees, J. Mol. Spectrosc. 293-294, 38 (2013)]. The CIs illuminate the role of geometric phase in methanol. More generally, they suggest the importance of energy transfer processes localized near the CIs.

Torsion–Inversion Tunneling Patterns in the CH-Stretch Vibrationally Excited States of the G12 Family of Molecules Including Methylamine

Two torsion-inversion tunneling models (models I and II) are reported for the CH-stretch vibrationally excited states in the G12 family of molecules. The torsion and inversion tunneling parameters, h(2v) and h(3v), respectively, are combined with low-order coupling terms involving the CH-stretch vibrations. Model I is a group theoretical treatment starting from the symmetric rotor methyl CH-stretch vibrations; model II is an internal coordinate model including the local-local CH-stretch coupling. Each model yields predicted torsion-inversion tunneling patterns of the four symmetry species, A, B, E1, and E2, in the CH-stretch excited states. Although the predicted tunneling patterns for the symmetric CH-stretch excited state are the same as for the ground state, inverted tunneling patterns are predicted for the asymmetric CH-stretches. The qualitative tunneling patterns predicted are independent of the model type and of the particular coupling terms considered. In model I, the magnitudes of the tunneling splittings in the two asymmetric CH-stretch excited states are equal to half of that in the ground state, but in model II, they differ when the tunneling rate is fast. The model predictions are compared across the series of molecules methanol, methylamine, 2-methylmalonaldehyde, and 5-methyltropolone and to the available experimental data.

An extended E⊗e Jahn-Teller Hamiltonian for large-amplitude motion: Application to vibrational conical intersections in CH3SH and CH3OH

An extended E⊗e Jahn-Teller Hamiltonian is presented for the case where the (slow) nuclear motion extends far from the symmetry point and may be described approximately as motion on a sphere. Rather than the traditional power series expansion in the displacement from the C3v symmetry point, an expansion in the spherical harmonics is employed. Application is made to the vibrational Jahn-Teller effect in CH3XH, with X = S, O, where the equilibrium CXH angles are 83° and 72°, respectively. In addition to the symmetry-required conical intersection (CI) at the C3v symmetry point, ab initio calculations reveal sets of six symmetry-allowed vibrational CIs in each molecule. The CIs for each molecule are arranged differently in the large-amplitude space, and that difference is reflected in the infrared spectra. The CIs in CH3SH are found in both eclipsed and staggered geometries, whereas those for CH3OH are found only in the eclipsed geometry near the torsional saddle point. This difference between the two molecules is reflected in the respective high-resolution spectra in the CH stretch fundamental region.

Effect of small molecules on the phase behavior and coacervation of aqueous solutions of poly(diallyldimethylammonium chloride) and poly(sodium 4-styrene sulfonate)

HYPOTHESIS Complex coacervates are capable of easily partitioning solutes within them based on relative affinities of solute-water and solute-polyelectrolyte pairs, as the coacervate phase has low surface tension with water, facilitating the transport of small molecules into the coacervate phase. The uptake of small molecules is expected to influence the physicochemical properties of the complex coacervate, including the hydrophobicity within coacervate droplets, phase boundaries of coacervation and precipitation, solute uptake capacity, as well as the coacervate rheological properties. EXPERIMENTS Phase behavior of aqueous solutions of poly(diallyldimethylammonium chloride) (PDAC) and poly(sodium 4-styrene sulfonate) (SPS) was investigated in the presence of various concentrations of two different dyes, positively charged methylene blue (MB) or non-charged bromothymol blue (BtB), using turbidity measurements. These materials were characterized with UV-vis spectroscopy, zeta potential measurements, isothermal titration calorimetry (ITC), fluorescence spectroscopy, and dynamic rheological measurements. FINDINGS The presence of MB or BtB accelerates the coacervation process due to the increased hydrophobicity within coacervates by the addition of MB or BtB. The encapsulated MB or BtB tends to reduce the ionic crosslink density in the PDAC-SPS coacervates, resulting in a much weaker interconnecting network of the PDAC-SPS coacervates.

Strategy for the Co-Assembly of Co-Axial Nanotube–Polymer Hybrids

Nanostructured materials having multiple, discrete domains of sorted components are particularly important to create efficient optoelectronics. The construction of multicomponent nanostructures from self-assembled components is exceptionally challenging due to the propensity of noncovalent materials to undergo structural reorganization in the presence of excipient polymers. This work demonstrates that polymer-nanotube composites comprised of a self-assembled nanotube wrapped with two conjugated polymers could be assembled using a layer-by-layer approach. The polymer-nanotube nanostructures arrange polymer layers coaxially on the nanotube surface. Femtosecond transient absorption (TA) studies indicated that the polymer-nanotube composites undergo photoinduced charge separation upon excitation of the NDI chromophore within the nanotube.

Vibrational conical intersections in CH3SH: implications for spectroscopy and dynamics in the CH stretch region

The adiabatic separation in methyl mercaptan of the high-frequency asymmetric CH stretch vibrations from the lowfrequency torsional (γ) and CSH bend (ρ) coordinates yields a set of 7 vibrational conical intersections (CIs). The three CIs in the staggered conformation at ρ = 79◦ are close to the global minimum energy geometries (ρe = 83.3◦), accounting for the observed near-degeneracy of the two asymmetric CH stretch vibrations. The vibrational frequencies were computed at the CCSD(T)/aug-cc-pVTZ level. A new high-order Exe Jahn-Teller model, which involves a spherical harmonic expansion in ρ and γ, fits the calculated electronic and vibrational energies over the whole range of γ and for ρ between 0◦ and 100◦ to within a standard deviation of 0.2 cm−1. The pattern of the CIs contrasts with that in methanol where the CIs occur only in the eclipsed conformation near the top of the torsional barrier. An examination of three alternative diabatization schemes for the two molecules points to rather different nuclear dynamics. In CH3SH crossings between the upper and lower adiabatic surfaces are predicted to occur predominantly with motion along the CSH bending coordinate; whereas in CH3OH, such crossings are predicted to occur predominantly with torsional motion.

VIBRATIONAL JAHN-TELLER EFFECT IN NON-DEGENERATE ELECTRONIC STATES

The Jahn-Teller theorem a states that “All non-linear nuclear configurations are therefore unstable for an orbitally degenerate electronic state.” In 1982, Kellman b realized that the Jahn-Teller theorem also applies to nonlinear molecular species in non-degenerate electronic states when there are high-frequency vibrations that are degenerate at a symmetrical reference geometry. When those high frequencies can be considered as adiabatic functions of degenerate low-frequency coordinates, there is a spontaneous Jahn-Teller distortion that lifts the degeneracy of the high-frequency vibrations. Kellman applied the vibrational Jahn-Teller (vJT) concept to the Van der Waals dimer (SF6)2. In this talk, the vJT concept is applied to E ⊗ e systems that are small bound molecules in non-degenerate electronic states. The first case considered in systems for which the global minimum of the electronic potential has C3v symmetry.For such systems, including (C6H6)Cr(CO)3 and CH3CN, the vJT effect leads to a significant splitting of the degenerate highfrequency vibrations (CH or CO stretches), but the spontaneous vJT distortion is exceptionally small. The second case in systems for which the global minimum of the electronic potential is substantially distorted from the C3v reference geometry. For the second case systems, including CH3OH and CH3SH, the vJT splitting of the degenerate CH stretches is much larger, on the order of several 10Äôs of cm−1). For both cases, there is the symmetry-required vibrational conical intersection at the C3v reference geometry. For the second case systems, there are additional symmetry-allowed vibrational conical intersections far from the C3v geometry but energetically accessible to the molecule at thermal energies. For both cases, the vibrationally adiabatic surfaces, including the multiple conical intersections, are well described by modest extensions to a high-order Hamiltonian that was developed for the electronic Jahn-Teller problem.c

Assignment And Analysis Of The No2 In-plane Rock Band Of Nitromethane Recorded By High-resolution Ftir Synchrotron Spectroscopy

The high-resolution rotationally resolved Fourier Transform Far-infrared spectrum of the NO2 in plane-rock band (440-510 cm−1) of nitromethane (CH3NO2) has been recorded using the Far-Infrared Beamline at the Canadian Light Source, with a resolution of 0.00096 cm−1. More than 1500 transitions lines have been assigned for m′ = 0; Ka ′ ≤ 7; J ′ ≤ 50; using an automated ground state combination difference program together with the traditional Loomis Wood approachb. Transitions involving m′ = 0; Ka ′ ≤7; J ′ ≤ 20; in the upper vibrational state are fit using the six-fold torsionrotation program developed by Ilyushin et.alc. The torsion-rotation energy pattern in the lowest torsional state ( m′ = 0) of the upper vibrational state is similar to that of the vibrational ground state.

SPECTRAL ASSIGNMENTS AND ANALYSIS OF THE GROUND STATE OF NITROMETHANE IN HIGH-RESOLUTION FTIR SYNCHROTRON SPECTRA

The Fourier Transform infrared spectra of CH3NO2, have been recorded, in the 400-950 cm−1spectral region, at a resolution of 0.00096 cm−1, using the Far-Infrared Beamline at Canadian Light Source. The observed spectra contain four fundamental vibrations: the NO2 in-plane rock (475.2 cm−1), the NO2 out-of-plane rock (604.9 cm−1), the NO2 symmetric bend (657.1 cm−1), and the CN-stretch (917.2 cm−1). For the lowest torsional state of CN-stretch and NO2 in-plane rock, transitions involving quantum numbers, m′′ = 0; J ′′ ≤ 50 and Ka ′′ ≤ 10, have been assigned with the aid of an automated ground state combination difference program together with a traditional Loomis Wood approachb. Ground state combination differences derived from more than 2100 infrared transitions have been fit with the six-fold torsionrotation program developed by Ilyushin et.alc. Additional sextic and octic centrifugal distortion parameters are derived for the ground vibrational state.