Charles M. Gabrys

Infrared spectroscopy of carbo-ions. VI: C-H stretching vibration of the acetylene ion C2H2+ and isotopic species

The infrared spectra of the band of the 2Π–2Π asymmetric hydrogen stretching vibration in the three isotopic acetylene ions C2H2+ (ν3), 13C2H2+ (ν3), and DCCH+ (ν1) have been observed and analyzed. The high resolution infrared spectra were recorded using a difference‐frequency laser spectrometer as the tunable coherent infrared source probing an ac glow discharge. Velocity modulation, noise subtraction, and unidirectional multipassing of the infrared beam through the discharge cell provided high sensitivity. C2H2+ was produced in a gas mixture of H2, He, and either CH4 or C2H2, with a total pressure of ≊7 Torr in multiple‐inlet–outlet air‐, water‐, and liquid‐nitrogen‐cooled discharge tubes; C2H2 freezing precluded its use in liquid‐N2‐cooled discharges. Complicated by a strong perturbation whose maximum occurred at N’=15 for F1 and N’=14 for F2, the assignment of the spectrum of normal C2H2+ was made possible by (1) fortuitous discharge conditions which provided unambiguous discrimination of C2H2+ lines ...

Observation of infrared forbidden transitions of H3

Abstract The ν 1 ← 0 and ν 1 + ν 2 ← ν 2 forbidden transitions of H 3 + have been observed. The former is induced by the rovibrational Birss resonance between the ν 1 and ν 2 states and the latter by Fermi resonance. The observation leads us to the determination of absolute values of vibration-rotation energy levels related to the ν 1 and the ν 1 + ν 2 states. The assignment of the spectrum was constantly helped by “first principles calculations” of Miller, Tennyson, and Sutcliffe. We give an interpretation of the spectrum also based on the traditional vibration-rotation formalism.

Observation of the 2ν2(l=2)←0 overtone band of H+3

The overtone band of 2ν2(l=2)←0 of H+3 in the 2 μm infrared region has been observed in absorption by using a difference frequency laser spectrometer. The LiIO3 nonlinear optical system assembled by Bawendi and the bidirectional multiple path optical arrangement with velocity modulation have enabled us to observe 34 vibrational–rotation transitions from 5094 to 4557 cm−1. Liquid‐N2 cooled plasmas were used for transitions involving low rotational levels, while gaseous‐N2 cooled plasmas were used for high rotational levels, up to J,K=(9,9). From the relative intensities of the transitions, rotational temperature has been estimated to be ∼270 K for the former and ∼1300 K for the latter. The Δ‖k−l‖=±3 selection rule of the overtone band allows us to determine the absolute energy level values by combining the frequencies of the overtone band with those of the fundamental and hot bands.

Solid state NMR measurements of conformation and conformational distributions in the membrane-bound HIV-1 fusion peptide.

The solid state NMR lineshape of a protein backbone carbonyl nucleus is a general diagnostic of the local conformational distribution in the vicinity of that nucleus. In addition, measurements of carbonyl chemical shifts and 2D exchange spectra provide information about the most probable conformation in the distribution. These types of solid state NMR methodologies have been applied to structural studies of the membrane-bound HIV-1 fusion peptide. This peptide is derived from a domain of the HIV-1 gp41 envelope protein, which is critical for viral-host cell-membrane fusion. Even in the absence of the rest of the envelope protein, the fusion peptide will fuse liposomes or erythrocytes. The solid state NMR measurements demonstrate that the center of the membrane-bound HIV-1 fusion peptide is structured, while the C-terminus is highly disordered. The structural distribution at the peptide center is lipid-dependent, with the greatest degree of structural homogeneity in a lipid environment whose composition reflects that of the target T cells. When bound to the lipid mixture, the peptide center is predominately beta sheet. The beta-sheet structure may be diagnostic of peptide oligomerization, which is thought to be a requirement for membrane fusion activity. Although the peptide partially disrupts bilayer orientational ordering in stacked glass-plate samples, 2H NMR demonstrates that the bilayers remain intact in the presence of the fusion peptide and are not micellized. The retention of the bilayer phase may relate to the biological requirement that the virus should fuse with, but not destroy, the target host cell membrane.

Spectral lines and distribution of H+3 in high rotational levels

Infrared spectrum of the ν2 fundamental band of H+3 in high rotational levels has been studied. Three motives for this study were (i) to provide laboratory data for the observation of astronomical objects in which a large abundance of H+3 exists at high temperature, (ii) to study kinetic energy distribution of H+3 in plasmas and to determine its rotational and translational temperatures, and (iii) to provide information on high rovibrational states for the variational calculations on the intramolecular dynamic of H+3. In order to increase the kinetic temperature, water‐cooled plasmas with helium dominated gas mixtures with He/H2∼5/0.6 torr were used. The observed rotational level in the ground state with the highest rotational quantum numbers was J=K=15 which has the rotational energy of 5091.6 cm−1. It was found from the observed relative intensities of the rovibrational transitions and linewidths that H+3 in the plasmas were in approximate thermal equilibrium with both rotational and translational tempe...

Laboratory infrared spectra of CH2D+ and HCCD+ and predicted microwave transitions

In order to provide radio-astronomical probes for CH 3 + and HCCH + , which play crucial roles in the chemical evolution of molecular clouds, infrared vibration-rotation spectra of their monodeuterated species have been measured. Rotational transitions of CH 2 D + and HCCD + are predicted

Infrared spectroscopy of carbo-ions: The ν1 and ν4 bands of CH2D+, and the ν1 band of CHD2+

Abstract The ν 1 and ν 4 bands of CH 2 D + and the ν 1 band of CHD 2 + have been observed and analyzed. Both species were formed concurrently in a gas mixture of CD 4 :CH 4 :H 2 :He::120:10:100:7000 m Torr discharged at 6 kHz ac with ∼ 150 mA in a multiple-inlet multiple-outlet, liquid-nitrogen-cooled cell. Their infrared vibration-rotation spectra were recorded with a difference-frequency spectrometer and detected by velocity modulation with unidirectional multipassing and noise subtraction of the probing infrared radiation. The transitions of each band were fitted separately to a Watson A -reduced Hamiltonian to determine the rotational and quartic centrifugal distortion constants. The determined band orgins (in cm −1 ) are ν 1 = 3004.7650(5) and ν 4 = 3105.8406(6) for CH 2 D + and ν 1 = 3056.1694(8) for CHD 2 + . The inertial defects Δ, centrifugal distortion constants, and rotational transitions have been calculated and are given for these two species.

Analysis of local conformation of membrane-bound and polycrystalline peptides by two-dimensional slow-spinning rotor-synchronized MAS exchange spectroscopy

Abstract2D slow-spinning, rotor-synchronized MAS exchange spectroscopy (SSRS-MASE) was applied to study local secondary structure of three structurally different peptides, two of which were membrane-bound. Each peptide was 13C carbonyl labeled at two adjacent residues in the peptide backbone. In general, this methodology is attractive for membrane-bound peptides because of its lenient spinning, decoupling, and RF homogeneity requirements.For a single set of raw SSRS-MASE data, two linearly independent methods exist for obtaining a 2D spectrum and each spectrum can be fit to obtain conformational constraints. An approach is described for combining the results of these two fits and this method is shown to work for spectra with both resolved and unresolved labeled site resonances. A spectrum is often fit well to a few different conformations which have somewhat different values of the fitting parameter χ2. A simple statistical theory is developed which relates the Δχ2 difference between a local minimum and the global minimum χ2 to the likelihood that the local minimum conformation is the correct structure. Because uncertainty in the simulated data can also contribute to the overall fitting uncertainty, an empirical method is described for incorporating the simulation uncertainty into the Δχ2 analysis.These data analysis methods were tested on polycrystalline Ala-Gly-Gly and then applied to the membrane-bound melittin and HIV-1 fusion peptides. Melittin gave a best-fit α helical structure at Ala-4 while the fusion peptide gave a good-fit β strand structure at Phe-8. The melittin analysis is in agreement with the known overall structure of this peptide.

Fine structure of the H2 5g–4f inter-Rydberg transition revealed by difference frequency laser spectroscopy

The spectrum of the 5g–4f inter-Rydberg band of H2 has been recorded with a difference frequency laser system and analyzed using multichannel quantum defect theory (MQDT). New transitions have been observed; in addition to the singlet–triplet splittings previously observed, the hyperfine structure of the ortho-hydrogen spectrum is partially resolved in the present experiment. MQDT is used to analyze the data in a two stage process. First, the ab initio MQDT predictions were refined by fitting the quantum defect functions over a range of internuclear separation R. Second, 4f singlet and triplet quantum defects are extracted from the para-hydrogen spectra, i.e., those lines without complicating hyperfine structure. This information was then used to calculate the fine structure of a sample ortho-hydrogen line, R3(2)v+=0. While the spectra are predominantly composed of absorption lines, some transitions from high vibrational levels of the 5g triplet manifold to 4f triplet levels are observed in stimulated emi...

High resolution infrared spectroscopy of deuterium in p‐H2 matrix

The high resolution spectrum of the fundamental band of deuterium impurity embedded in solid parahydrogen has been studied using a difference frequency laser spectrometer. This observation represents the first case of high resolution matrix spectroscopy in solid hydrogen. Both the Q1←0(0) and Q1←0(1) transitions were resolved into sharp features with a typical linewidth of ≲ 10 MHz, while both the S1←0(0) and S1←0(1) transitions appeared to be much broader with widths of ∼0.07 cm−1. A discussion of the linewidths based on homogeneous broadening is presented.