Dimers, rotational temperature

The three bands in Figure 9.46 show resolved rotational stmcture and a rotational temperature of about 1 K. Computer simulation has shown that they are all Ojj bands of dimers. The bottom spectmm is the Ojj band of the planar, doubly hydrogen bonded dimer illustrated. The electronic transition moment is polarized perpendicular to the ring in the — Ag, n — n transition of the monomer and the rotational stmcture of the bottom spectmm is consistent only with it being perpendicular to the molecular plane in the dimer also, as expected. 

L. Woste The rotational temperature of dimers in our molecular cluster beam was measured to be about 7 K. So only very few rotational states up to about J = 9 are significantly populated. 

The development of supersonic molecular beam techniques created new opportunities to study tunneling effects in gas-phase isolated molecules and dimers at ultralow translational and rotational temperatures. Modern low-temperature chemistry, therefore, includes the study of chemical dynamics of molecules in various states of aggregation. 

Determination of activation energy for barrier of internal rotation. - Temperature dependence of oih(2) shown in a figure of [75Watl]. [74Watl] Determination of association energy from radicals to dimer. [65Kuhl, 74Watl] In benzene. 

Figure 4.8 Vibrational, translational and rotational temperatures of carbon dimers emitted successively from thermally excited fullerene cations, as predicted by phase space theory with quantum densities of states.

Fourier transform techniques have become very powerful, particularly when used with pulsed sources of the sample. They are ubiquitous in studies of van der Waals molecules, in which two or more entities are very weakly bound together in the gas phase, such as HCT Ar, but are also used to determine structures of many other compounds, of which the mixed alkali halide dimer LiNap2 is an example. In a typical experiment, the sample is introduced into the cell in a pulsed supersonic jet expansion, the rotational temperature being reduced to a few Kelvin. A pulse of microwave radiation then aligns the dipole moments of the molecules, so that the sample is polarized on a macroscopic scale. The subsequent decay of this polarization is recorded, and Fourier transformation of the free induction decay yields the spectrum [9]. 

D rot and yib are the contributions of rotational and vibrational degrees of freedom, respectively, to the molar heat capacity at constant volume. The rotational contribution to the heat capacity can be considered classical, because T 0rot. where drot is the characteristic rotational temperature, and the dimer is assumed to be a rigid rotor - a hypothesis commensurate with the use of the Eucken approximation. Hence,... 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

The geometry of the zwitterions with its exocylic out-of-plane methylene group was quasi-preserved in the recently reported dibenzodioxocine derivative (18) that was formed in rather small amounts by rapidly degrading the NMMO complex at elevated temperatures.45 Strictly speaking, dibenzodioxocine dimer 18 is actually not a dimer of ortho-quinone methide 3, but of its zwitterionic precursor or rotamer 3a (Fig. 6.17). As soon as the out-of-plane methylene group in this intermediate rotates into the ring plane, the o-QM 3 is formed irreversibly and the spiro dimer 9 results... 

The dimer (101) is slowly formed from bis(trimethylsilyl)aminotrimethylsilyl-iminophosphine on standing at room temperature. The stereochemistry of (101) appears to be fixed, due to a very high (AGfc > 27 kcal mol-1) P—N rotational barrier.77... 

Nitrosobenzene was studied by NMR and UV absorption spectra at low temperature146. Nitrosobenzene crystallizes as its dimer in the cis- and fraws-azodioxy forms, but in dilute solution at room temperature it exists only in the monomeric form. At low temperature (—60 °C), the dilute solutions of the dimers could be obtained because the thermal equilibrium favours the dimer. The only photochemistry observed at < — 60 °C is a very efficient photodissociation of dimer to monomer, that takes place with a quantum yield close to unity even at —170 °C. The rotational state distribution of NO produced by dissociation of nitrosobenzene at 225-nm excitation was studied by resonance-enhanced multiphoton ionization. The possible coupling between the parent bending vibration and the fragment rotation was explored. 

The dynamic behavior of various solid organolithium complexes with TMEDA was investigated by variable-temperature and CP/MAS and Li MAS NMR spectroscopies. Detailed analysis of the spectra of the complexes led to proposals of various dynamic processes, such as inversion of the five-membered TMEDA-Li rings and complete rotation of the TMEDA ligands in their complex with the PhLi dimer (81), fast rotation of the ligands in the complex with cyclopentadienyllithium (82) and 180° ring flips in the complex with dilithium naphthalene (83) °. The significance of the structure of lithium naphthalene, dilithium naphthalene and their TMEDA solvation coiMlexes, in the function of naphthalene as catalyst for lithiation reactions, was discussed . ... 

Fig.la shows the first 200ps of the fs DFWM spectrum of HCOOH vapor at room temperature. A complete fitting of the spectrum delivers values of rotational and CD constants and information on the diagonal elements of the PT for HCOOH. The rotational and CD constants are in good agreement with reference values from a Fourier-Transform microwave experiment. [7] The experimental spectrum is fitted very well by the simulation (Fig. 2b), except for the region 120-130ps (marked in Fig. 1). The spectral features in this region originate from the formic acid dimer (HCOOHh-...

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