Evan G. Robertson

Hydration of neurotransmitters: a computational and spectroscopic study of a noradrenaline analogue, 2-amino-1-phenyl-ethanol

Singly and multiply hydrated clusters of the noradrenaline analogue, 2-amino-I-phenyl-ethanol (APE) have been studied using a combination of resonant two-photon ionization time of flight spectroscopy (R2PI-TOF), infrared ion-dip spectroscopy and ab initio quantum chemical calculation. Singly hydrated clusters populate two distinct structures: the water molecule either hydrogen-bonds to the ethanol group in the extended AG conformer (leaving the intramolecular OH → N hydrogen bond intact) or inserts into the intramolecular hydrogen bond in the (distorted) ethanolamine side chain (promoting a weak NH → O bond). The observed doubly and triply hydrated clusters both display insertion structures only, with the water molecules arrayed as linear chains, hydrogen bonded to the functional groups of the side-chain and again promoting a weak NH → O bond along the distorted ethanolamine side-chain. The infrared spectrum of the 1:4 cluster of APE, which is very similar to that of the corresponding cluster of ephedrine, includes new features in the ‘window region’ (3500–3700 cm−1), indicating the onset of a three-dimensional assembly. Comparisons with ab initio computed spectra favour a structure that incorporates a cyclic water tetramer linked to the two functional groups on the ethanolamine side-chain.

Completing the picture in the rovibrational analysis of chlorodifluoromethane (CHClF2): v3 and v8

Abstract Collisionally cooled (∼150K) and room temperature high resolution FTIR spectra of chlorodifluoromethane have been combined in an extensive rovibrational analysis of the Coriolis-coupled band system ν 3 and ν 8. A total of 5525 lines have been assigned to CH35ClF2, with J max=95 and K max=54. Local resonances that perturb three separate regions of the spectrum have been accommodated by including in a least-squares fit the ‘dark’ states ν 9=3 and ν 9=2; ν 6=1. The ν 9=3 levels interact with ν 3=1 via both a- and c-axis Coriolis terms while ν 9=2; ν 6=1 interacts with ν 8=1 via a c-axis Coriolis term. The overall rms error for the transitions is 0.000 345cm−1, and the fitted dark states rotational constants are within 0.1% of calculated values based on ν 6 and ν 9 vibration-rotation constants. Six spectral lines of 3ν 9, observed through borrowed intensity from ν 3, have been assigned to confirm the analysis. Spectral analysis by subtraction of simulated intensities (SASSI) was also used to enable the assignment of 4494 CH37ClF2 lines, with J max=74 and K max=42. The dipole moment ratios of the a/c-hybrid, ν 3 and b-type, ν 8 bands are .

Reading between the lines: Exposing underlying features of high resolution infrared spectra (CHClF2)

A technique to facilitate the assignment of congested high resolution IR spectra is presented. It involves subtraction of component spectra simulated from known spectroscopic constants. Application to the ν2 (1313.1 cm−1) and ν7 (1351.7 cm−1) bands of CHClF2 led to rovibrational line assignments for less intense and overlapping features including the minor isotopomer CH37ClF2 and c-type lines of CH35ClF2. The number of transitions assigned to the fundamental bands increased from 1721 to 6031 for 37Cl, and from 7496 to 9664 for 35Cl, with a greater range of rotational quantum numbers than found in a previous study. A total number of 1457, 484 and 262 line assignments were made for 210911, 710911 and 210611 hotbands respectively, features otherwise completely obscured in the observed spectrum. Rotational and centrifugal distortion parameters were fitted for all the upper states.

A comparison of predicted and experimental vibrational spectra in some small fluorocarbons.

The vibrational assignments of difluoromethane, cis and trans 1,2‐difluoroethylene, trifluoroethylene, 2‐fluoropropene, 1,1‐difluoroethylene, and tetrafluoroethylene have been reassessed in the light of recent theoretical work. Inconsistent experimental wave number values and assignments have been clarified, and some changes to the experimental assignments are proposed. The assignments compare favorably with recent scaled density functional theory calculations using the hybrid three‐parameter B3‐PW91 density functional.

High-resolution FTIR spectroscopy of chlorodifluoromethane: ν2 and ν7

Abstract High-resolution FTIR spectra of chlorodifluoromethane (R22) were measured both at room temperature and cooled to approximately −100 °C in a collisional cooling cell. A rovibrational analysis was performed for ν 2 , the a/c-hybrid band at 1313 cm −1 and ν 7 , the b-type band at 1351 cm −1 . 7400 and 1700 lines were assigned to CH 35 ClF 2 and CH 37 ClF 2 , respectively, with quantum numbers up to J =98 and K a =46. Effective constants to the sextic level have been fitted using Watsons A-reduction Hamiltonian. More accurate spectroscopic constants were obtained by fitting the two states simultaneously, taking into account both first- and second-order c -axis Coriolis interactions between the two bands. The ν 2 band is predominantly a-type, but weaker c-type transitions assigned for CH 35 ClF 2 enable the a/c-hybrid character ( μ a 2 / μ c 2 ) to be determined as 5.76.

Conformational analysis by laser spectroscopy in the gas phase: p-methoxyphenethylamine (a neurotransmitter)

Rotationally resolved fluorescence excitation spectra of seven different S1 ← S0 origin bands of p-methoxyphenethylamine have been obtained in the collision-free environment of a molecular beam. Analyses of these spectra permit unambiguous assignments of specific bands to specific conformers, based on differences in their inertial defects, rotational constants, and electronic transition moment orientations, thereby resolving a controversy in the recent literature.

Tetrafluoroethylene: high resolution FTIR spectroscopy

High resolution FTIR spectra of tetrafluoroethylene (TFE) were measured in a cell cooled to approximately 160 K. Rovibrational analyses of ν9, the b-type band at 1339.9 cm−1 and ν11, the a-type band at 1187.6 cm−1 led to assignment of 2691 and 3101 lines respectively, with Jmax = 87. Ground state combination differences generated from these transitions were fitted to give the first spectroscopically derived rotational constants for this molecule (A0 = 5512.790 ± .011 MHz, B0 = 3237.899 ± .028 MHz, C0 = 2038.495 ± .006 MHz). An MP2/TZV + (3df) structure, with rCC = 132.52 pm, rCF = 131.43, ∠CCF = 123.273°, gives rotational constants within 0.12% of the estimated re constants. Fitted ground state centrifugal distortion constants are in excellent agreement with those calculated from ab initio harmonic force fields. Rovibrational spectroscopic constants were also derived for ν9 and ν11. Fitting the rotational levels of ν9 required treatment of a dark state that gives rise to local perturbations through a c-axis Coriolis interaction.

High resolution FTIR spectroscopy of CFC's in a supersonic jet expansion

Abstract High resolution FTIR spectra of dichlorodifluoromethane and bromochlorodifluoromethane have been recorded on a Bruker HR120 spectrometer coupled to a supersonic jet nozzle apparatus. Rotational temperatures of 40K for Freon-12 and Halon-1211 have been achieved. An analysis of the v1 band of Freon-12 has been carried out.

Resonant 2-photon ionization study of the conformation and the binding of water molecules to 2-phenylethanethiol (PhCH2CH2SH)

The structures of 2-phenylethanethiol (PET, PhCH(2)CH(2)SH) and its 1:1 water clusters have been studied using resonant two-photon ionization spectroscopy including band contour analysis and UV-UV holeburning, combined with extensive ab initio calculations on ground and excited states. The most populated conformer, labeled Ggpi, has a gauche arrangement about the SCCC and HSCC bonds that permits a stabilizing SH...dpi type of hydrogen bond. The other observed conformer, Ag, is anti with respect to the SCCC bond. In the dominant 1:1 water cluster, a water molecule binds to the Ggpi conformer via an OH...S hydrogen bond and two significant CH...O interactions. There is also evidence for water binding to conformer Ag with a similar arrangement, and for a second Ggpi cluster where water inserts between the SH and the aromatic ring. The additional interactions to the water molecules result in net D(e) binding energies approximately double those resulting from a single thiol-water hydrogen bond. The (1)(pi,pi(*)) excited state lifetimes in the bare molecules are very short because of internal conversion to a dissociative (1)(n,pi(*)) state related to the thiol. In the dominant Gw(1) cluster, the lifetime is significantly increased from <1 to approximately 4 ns. Hydrogen bonding to the thiol, which raises the energy of the dissociative (1)(n,pi(*)) state, accounts for this behavior.

Surface-enhanced raman scattering spectroscopy of resveratrol.

We report here, for the first time, the surface-enhanced Raman scattering (SERS) spectra of resveratrol using KNO3-aggregated citrate-reduced silver (Ag) colloids. The technique provided a substantial spectral enhancement and therefore good quality spectra of resveratrol at parts per million (ppm) concentrations. The detection limit was found to be <1 μM, equivalent to <0.2 ppm. The SERS profile additionally closely resembled its normal solid-state Raman spectrum with some changes in relative intensity. These intensity changes, together with a precise band assignment aided by density functional theory calculations at the B3LYP/6–31G(d) level, allowed the determination of the structural orientation of the adsorbed resveratrol on the surface of the metal nanoparticles. In particular, the SERS spectra obtained at different resveratrol concentrations exhibited concentration-dependent features, suggesting an influence of surface coverage on the orientation of the adsorbed molecules. At a high concentration, an adoption of close-to-upright orientation of resveratrol adsorbed on the metal surface through the p-OH phenyl ring is favoured. The binding structure is, however, altered at lower surface coverage when the concentration decreases to a tilted orientation with the trans-olefin C=C bond aligning closer to parallel to the surface of the Ag nanoparticles.