Single Resonance Spectra Research Articles

Review Article: The weak interactive characteristic of resonance cells and broadband effect of metamaterials

Metamaterials are artificial media designed to control electromagnetic wave propagation. Due to resonance, most present-day metamaterials inevitably suffer from narrow bandwidth, extremely limiting their practical applications. On the basis of tailored properties, a metamaterial within which each distinct unit cell resonates at its inherent frequency and has almost no coupling effect with the other ones, termed as weak interaction system, can be formulated. The total response of a weak interaction system can be treated as an overlap of the single resonance spectrum of each type of different unit cells. This intriguing feature therefore makes it possible to accomplish multiband or broadband metamaterials in a simple way. By introducing defects into metamaterials to form a weak interaction system, multiband and broadband electromagnetic metamaterials have first been experimentally demonstrated by our group. The similar concept can also be readily extended to acoustic and seismic metamaterials.

Infrared-infrared double resonance spectroscopy of cyanoacetylene in helium nanodroplets.

Infrared-infrared double resonance spectroscopy is used as a probe of the vibrational dynamics of cyanoacetylene in helium droplets. The nu1 C-H stretching vibration of cyanoacetylene is excited by an infrared laser and subsequent vibrational relaxation results in the evaporation of approximately 660 helium atoms from the droplet. A second probe laser is then used to excite the same C-H stretching vibration downstream of the pump, corresponding to a time delay of approximately 175 micros. The hole burned by the pump laser is narrower than the single resonance spectrum, owing to the fact that the latter is inhomogeneously broadened by the droplet size distribution. The line width of the hole is characteristic of another broadening source that depends strongly on droplet size.

Multiple timescales in the intramolecular vibrational energy redistribution of highly excited methanol

Highly resolved vibrational overtone spectra of jet-cooled CH3OH, obtained using selective IRLAPS detection of the vibrational transition, reveal rich vibrational structure that carries detailed information on the intramolecular dynamics. Low-resolution single resonance spectra of the jet-cooled molecules exhibit a 50 cm–1 splitting resulting from strong coupling between the OH and CH stretch modes at the energy of the 5νOH band. Subsequent to coherent excitation, this coupling would lead to energy redistribution between the OH and CH on a 100 fs timescale. High-resolution spectra obtained using double-resonance excitation reveal smaller vibrational splittings resulting from weak vibrational couplings that control the longer time dynamics. The extensive vibrational structure in this overtone spectrum of methanol clearly indicates that it is the presence of low-order resonances rather than the total density of vibrational states that control the first 10 ps of vibrational-energy redistribution.

13C nuclear magnetic resonance spectra of some bromo- and methyl-substituted cyclohexa-2,5-dienones

The 13C n.m.r. spectra of cyclohexa-2,5-dienones produced by bromination or nitration of a variety of methyl-substituted phenols are reported. Most of the signals have been assigned with the assistance of selective decoupling. Correlations of chemical shifts resulting from replacement of hydrogen by methyl and by bromine in the positions originally ortho and meta to the hydroxy group have been attempted. The effects resulting from changing the substituents at the 4-position from Br, Br to Me, Br to Me, NO2, and to Me, OH have also been analysed. Comparison is made with effects found in related systems. Single-resonance spectra have been used to determine coupling constants {J(1H, 13C)}, and these also are compared with cognate results.

A combined study of vicinal and geminal carbon‐proton spin‐spin coupling constants as a tool in conformational analysis. The conformation of 2,6‐dichloro‐1,4‐oxathianes

Abstract13C FT single resonance spectra are obtained for two isomers of 2,6‐dichloro‐1,4‐oxathiane; one of the isomers is a racemic mixture. From a combined study of the various long‐range carbon‐proton coupling constants the conformations of both isomers could be determined. Each isomer exists in two conformers which are in a rapid interconversion equilibrium. Contrary to predictions based on the “anomeric‐effect”, cis‐2,6‐dichloro‐1,4‐oxathiane in the liquid phase is predominantly in the cis‐di‐equatorial chair form.

Heteronuclear double resonance: Individual overhauser effects in coupled AX 2 and AX 3 spectra

We describe a new way of investigating relaxation in multispin systems. In a double-resonance experiment, if the decoupler is off while the spectrum is collected then the line positions are the same as those in the single-resonance spectrum but the intensities are perturbed. This is due to population changes induced by the rf irradiation. A theory of these effects is developed which is free of many of the usual restrictive assumptions. Individual Overhauser effects for each line in the A part of an AX 2 or AX 3 spectrum are defined and can be calculated. In the calculation, all the correlations in the dipolar relaxation are included. The Overhauser effects for the three lines in the carbon spectrum of a methylene group are calculated as a function of decoupler offset for a particular decoupler power. These results are found to be in good agreement with experimental work on the central methylene group in diethyl malonate. The experiment complements longitudinal relaxation measurements and provides a good way of selectively probing relaxation mechanisms.

“Spin-tickling” in nuclear magnetic single resonance

An effect, not unlike “spin-tickling” in nuclear magnetic double resonance, can occur in single resonance spectra if there is a fortuitous proximity of particular spin states. Observation of the perturbation allows the assignment of relative signs of coupling constants; the perturbation may be chemically manipulated by changing solvents.

Applications of double resonance and Fourier transform n.m.r. spectroscopy in organic chemistry

Abstract

Determination of Ion Transit Times in an Ion Cyclotron Resonance Spectrometer

A method for directly determining ion transit times in an ion cyclotron resonance spectrometer has been developed which employs a simple combination of pulsed ion formation and time dependent trapping conditions. The variation of transit times with the adjustable parameters associated with the experiment (magnetic field strength, drift voltage, trapping voltage, pressure, and ion kinetic energy) is examined and found to be in quantitative agreement with the predictions of electrodynamic theory. The accurate determination of ion transit times facilitates the calculation of ion-molecule reaction rate constants of increased credibility. In addition, a technique for recording single resonance spectra by trapping voltage modulation is presented.

Relaxation effects in the nuclear magnetic double resonance spectra of a symmetrical three spin system (AB2)

Relaxation effects are studied in the steady-state proton-proton double resonance spectra of a symmetrical three spin system of the type AB2, formed by the ring protons of 2,6-dibromoaniline. The experiments were performed on a Varian HR-100 high resolution N.M.R. spectrometer suitably modified for field-frequency locking and added by the usual equipment for the purpose of frequency-sweep double resonance. The sample used is a 20 per cent solution in CCl4, purified and sealed in vacuum. The parameters describing the single resonance spectrum of the molecule were obtained to be and J AB=7·95±0·1 hz. Double resonance spectra obtained by irradiating each of the eight transitions in the single resonance spectrum, show significant relaxation effects for amplitudes of irradiation varying from 0·1 hz to 9·0 hz. These features were analysed using the density matrix theory of double resonance. The inhomogeneity of the applied magnetic field is found to be an important factor in the interpretation of the relaxation...

Effect of relaxation on the nuclear double resonance spectra of weakly coupled spin systems; proton double resonance in ethyl fluoride

The density-matrix method of analysing relaxation effects in nuclear magnetic double resonance experiments is extended to the case of large molecules containing several groups of equivalent nuclei giving rise to fist-order single resonance spectra. The difficulty of using this theory for such systems possessing degenerate energy levels is circumvented by assuming that the relaxation in the system occurs through an istropic random field with complete correlation between interactions within each group and no correlation between interactions with different groups. This relaxation mechanism preserves the equivalence of the spins in each group signified by the spin Hamiltonian. This assumption, along with those used in the general theory, not only permits, in suitable cases, the use of a convenient approximation originally proposed by Bloch for simplifying the computations involved in the theory, but also separates the problem into a linear superposition of several independent sub-problems of much smaller dime...

The NMR spectra of some halopropenes

The relative signs and magnitudes of the coupling constants, and the chemical shifts are given for the cis and trans 1-, and the 2-fluoro-, chloro-, and bromopropenes. The relative signs of all of the couplings in these molecules are determined by inspection of the overlap of first-order multiplets, by analysis of second-order features in single resonance spectra, and by proton-proton and proton-fluorine double resonance experiments. In these molecules, the geminal and long-range proton-proton couplings are opposite in sign to the vicinal, cis and trans proton-proton couplings, and are opposite in sign to all the proton-fluorine coupling constants.

The analysis of complex nmr spectra by multiple resonance techniques

The complete analyses of complex proton nmr spectra by double and triple resonance techniques are illustrated for AA′BB′MM′XX′ and ABCDMXY systems. The single resonance spectra of cis-4-cyclohexene-1,2-dicarboxylic anhydride and 1-methyl- cis-4-cyclohexene-1,2-dicarboxylic anhydride consist of so many overlapping multiplets that individual features are unrecognizable. However, by multiple resonance techniques it is possible to obtain good estimates for all the coupling constants and chemical shifts for these molecules.