Spectroscopy at low temperatures

Kummer S, Basche T and Brauchle C 1994 Terrylene in p-terphenyl a novel single crystalline system for single molecule spectroscopy at low temperatures Chem. Phys. Lett. 229 309-16... 

The two trimethylstannyl groups are crystal- 210 lographically equivalent, however, the three methyl groups are rotationally and vibrationally different, as shown by INS spectroscopy at low temperature. 

One is the steric effect. As in the case of amides, the E conformation is disfavored by the steric effect when R is equal to or larger than a methyl group. The second factor is the dipole moment. The E conformation is much more polar than the Z conformation. Because of these factors, esters usually assume the Z conformation and no E conformation is observed. To observe the latter, a special probe is needed. From the foregoing discussion, it is apparent that if R is hydrogen and R a large group, the E conformation might be present. Thus the E form of tm-butyl formate was detected by H NMR spectroscopy at low temperature (84). The barrier was found to be ca. 11 kcal/mol (85). 

MAS = H-MAS NMR spectroscopy at 298 K, ADLF = nuclear quadrupole resonance by level crossing (at 77 K), QE = quadrupole echo (static) NMR spectroscopy at low temperature of die solid state, p.wave = microwave spectroscopy of the gaseous state. 

P-Donor Ligands. and n.m.r. studies of tertiary phosphine adducts of TiCl4 in dichloromethane solution suggest the existence of fast, temperature-dependent equilibria involving the 1 1 (trigonal-bipyramidal) and 1 2 cis-octahedral) TiCl4-PR3 adducts and free PR3 (R = Me, Et, Pr, Bu, Ph, or cyclohexyl). Several of these 1 1 and 1 2 adducts have been isolated for the first time and characterized by analysis and i.r. spectroscopy. At low temperatures (cfl. 210 K) there is some evidence for the formation of dimeric species, formulated as [ClaTiCljTiClaPRa]. ... 

Carbonyl oxides are usually highly unstable and can be detected only by matrix isolation spectroscopy at low temperature or by FFP techniques. However, they can be stabilized through steric protection. Dimesityl ketone oxide (80c, ),niax = 398 nm) generated by the reaction of triplet dimesitylcarbene (19c) with O2 at... 

Razskazovskiy et al. employ ESR spectroscopy at low temperatures to investigate electron transfer within brominated DNA [8]. The brominated DNA base electron traps were introduced by bromination of DNA in ice-cooled aqueous solution. The procedure is shown by NMR and GC/MS techniques to modify thymine, cytosine, and guanine, transforming them into 5-bro-mo-6-hydroxy-5,6-dihydrothymine, T(OH)Br, 5-bromocytosine, CBr, and 8-bromoguanine, GBr, derivatives. The bromination products formed in molar ratio close to T(OH)Br/CBr/GBr 0.2 1 0.23 and serve as internal electron scavengers on y-irradiation. Structurally the CBr and GBr are planar, but T(OH)Br is quite nonplanar with the bromine directly above the molecular plane. This disrupts the DNA base stack. Paramagnetic products that result... 

In addition to ESR spectroscopy, which is a general method for detecting radicals, Dole et al. (9, 10, 11, 12) have developed a method of ultraviolet spectroscopy at low temperatures, which is specific for allylic and polyenylic radicals. Numerous papers have dealt with changes in polymers on irradiation, and all of these conclude that the reactions, in one way or another, arise from the formation of free radicals. Only a few papers describe experiments in which the radicals have been observed directly by ESR or ultraviolet spectroscopy at low temperatures. This article merely summarizes the present knowledge of the nature of radicals formed in polyolefins by irradiation in vacuum (ionizing radiation and ultraviolet light) and discusses some new trends in studying these radicals. 

The reaction sequence of Eq. 23-37 can be slowed by lowering the temperature. Thus, at 70K illumination of rhodopsin leads to a photostationary state in which only rhodopsin, bathorhodopsin, and a third form, isorhodopsin, are present in a constant ratio.510 Isorhodopsin (maximum absorption at 483 nm)513 contains 9-ds-retinal and is not on the pathway of Eq. 23-37. Resonance Raman spectroscopy at low temperature supports a distorted all-frans structure for the retinal Schiff base in bathorhodopsin.510 The same technique suggests the trans geometry of the C = N bond shown in Eqs. 23-38 and 23-39. Simple Schiff bases of 11-cz s-retinal undergo isomerization just as rapidly as does rhodopsin.514... 

Absolute rate constants for intramolecular reactions of amidyl radicals have been determined by ESR spectroscopy at low temperature and extrapolated to 27°C (82JA6071). The rate constants for intramolecular 1,5-hydrogen atom abstraction, kMs, from alkyl and acyl side chains are 1 x 105 and 4 x 104 s 1, respectively. If the C-5 hydrogen on the acyl... 

Excited states of Cr + complexes were explored by single crystal spectroscopy at low temperatures. In the dimeric [a Cr(0H)2Cra ] + the sharp 2E single excitations were used to determine orbital exchange parameters. Out-of-plane interactions are dominant. The complex CrCljt was studied in two exactly octahedral crystal environments. 

The reaction of bis- and of tris- thiopyrylium cations 177 and 178 with CD3ONa in CD3OD has been studied by H NMR spectroscopy. At low temperature, kinetically controlled mixtures of 2H- and/or AH- adducts were detected, but at room temperature the mixture equilibrated to afford the 2//-adducts exclusively <2005JOC6422>. 

Section I) and nuclear derivatives, which treat globally all the excited electronic transitions. In this subsection, we consider only Raman spectroscopy at low temperatures,47-79 as illustrated in Fig. 2.16. 

Piperidines and related systems have a number of (potentially) readily accessible conformations. In nipecotic acid 141 a hydrogen bond stabilizes the axial conformation 142. Both conformations have been detected by H NMR spectroscopy at low temperature and their spectra fully documented together with the effect of solvent and pH <2000J(P2)2382>. 

The vinyl cations were characterized by H, C, and Si NMR spectroscopy at low temperatures. Fig. 1 shows as a typical example the NMR spectrum of the l-mesityl-2-(triisopropyl)silyl vinyl cation 3. 

The /u-peroxo dimer intermediate, C, in the oxidation of four-coordinate Fe porphyrins to the PFe-O FeP /U.-OXO dimer via equations (23A-C) (cf. Scheme 2) of tetrakis(m-tolyl)porphrnatoiron(III), has been detected by NMR spectroscopy at low temperatures. The /r-peroxo complex has a larger isotropic shift (16 vs. 13 ppm for the ii-0x0 dimer at -50 °C) for the pyrrole-H at all temperatures, and has an antiferromagnetic coupling constant, —27, that is about 30% smaller than that for the /r-oxo dimer, E the magnetic moment decreases from 2.6/tb at —50°C to about 2.2/tb at —80°C, both consistent with weaker antiferromagnetic coupling in the /u-peroxo complex. ... 

Model heme complexes containing the ferryl unit and a nonoxidized porphyrin ring have been prepared and investigated by H NMR spectroscopy at low temperatures by Balch and La Mar and coworkers and by resonance Raman spectroscopy by Nakamoto and coworkers. The ferryl species have been produced by several means, including... 

The flash photolysis of H3 StX (X = Cl, Br or I) gives rise to HXSi , the structures of which have been investigated by emission and absorption spectra. Like the X2Si species they have a singlet ground state with a relatively small angle at silicon. Photolysis of HsSiNs at 193 nm in an argon matrix at lOK leads to the formation of HSi N that can be characterized by IR and UV spectroscopy at low temperature... 

ESR (electron spin resonance) and optical absorption spectroscopy at low temperatures were used to analyse the individual reaction steps of the optical and thermal polymerization reactions and their kinetics. The reaction steps are the photoinitiation, the chain propagation and chain termination reactions. 

NO on the basis of NMR and IR spectroscopy. At low temperatures, an unstable complex with /no at 1670 cm S assigned to the bent nitrosyl of (39), was observed. By monitoring the NMR spectrum at room temperature, researchers observed a signal they assigned to the N-methyl group of (40), and rj -nitrosomethane complexes have been isolated and structurally characterized (21 i). This complex then decomposes, either by a binuclear mechanism or by unimolecular elimination of methylnitrene, to give azomethane and the resulting oxocomplex (41). 

The intensive but almost hidden line at 706 nm, the N4-line is due to the exchange-coupled Cr ions (emission of Cr " pairs, like the R line, changes its place only a little in the spectra of different spinels and many other crystals). The origin of this line from the Cr pairs is deduced from lifetime measurements and time-resolved spectroscopy at low temperatures ... 

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