The Hydrogen Fluoride System

Hydrogen fluoride dimer, like with the water dimer, is one of the most frequently studied complexes. Both extensive theoretical and experimental data are available for analysis. The PES of the hydrogen fluoride dimer (see Figs. 15 and 16), if compared with that of water, is simpler two stationary points exist the quasi-linear structure (see Fig. 15) and the cyclic structure (see Fig. 16). 

The stationary structures obtained are consistent with literature and experimental results. Our best estimate of the dimerisation energy for the global minimum in its quasi-linear structure (Fig. 15) from the MO-VB calculations (4.51 kcal/mol) agrees well with the experimental value (4.54 kcal/mol, Ref. 16) and with the calculated values [51]. 

Work is in progress in our laboratory to provide an accurate intermolecular potential surface for molecular dynamics studies of liquid hydrogen fluoride. 

As another example, the potential surface of the He-CH4 complex (studied in Ref. lOe) is described. In these calculations a basis set 7s6p3dlflg on the C atom, 4s3pld on the H atoms, and 8s4p2dlf on the He atom, consisting of a total 182 functions, was adopted. The s and p functions were optimised so as to reproduce energies close to the Hartree-Fock limit of the CH4 molecule and He atom, respectively. The exponents of high-order polarisation functions were determined by maximising directly the dispersion contribution. 

A new accurate fitting of the calculated PES was realised by a Newton-Raphson procedure adopting the same potential form as used by Buck et al. [65]. The agreement between the MO-VB ab initio potential and the Buck potential, determined by direct fitting of the experimental data, is very satisfactory. 

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The hydrogen fluoride/ melamine agent (particularly when used as a two-phase system with pentane or carbon tetrachloride) gives alkyl fluorides from alkenes in almost quantitative yield.40,41,65 The following procedure is superior to others in its ready separation of the fluorine compounds and the repeated use of the hydrogen fluoride system.41... 

The experimental bond length for the hydrogen fluoride molecule is 0.917A. Determine the basis set required to predict this structure accurately. Perform your optimizations at the MP4 level of theory (electron correlation is known to be important for this system). 

Systems The Hydrogen Fluoride Dimer , J. Chem. Phys., 101, 9793. 

Other methods of excitation are effective or necessary for certain gain media. For example, certain energetic chemical reactions produce molecules in excited states. These excited molecules may then comprise the upper laser level of an inverted-population system. A specific example is the hydrogen fluoride "chemical laser" wherein excitation is provided by the reaction of hydrogen gas with atomic fluorine. Another method of excitation is simply the passage of an electric current through a semiconductor device. This serves as the exciter for diode lasers. 

The two most widely accepted hydrogen fluoride systems, besides pure hydrogen fluoride, are 70% hydrogen fluoride/pyridine (Olah s reagent)10 11-31-33 and tricthylaminetris(hydrogen fluoride).9 32 33-316 Tetrahydrofuran. various other amines, amides, carbamic acids and esters. 

Finally, aryltriazenes are excellent sources for the introduction of fluoride via arenediazonium ions using hydrogen fluoride systems.152 The triazene route is considered to be most suitable for introducing 18F into an aromatic ring of bioactive organic compounds, because it is a rapid, mild reaction and has the potential of giving high radiochemical yields.11 Examples are described in refs 153 and 154. 

Substituted 2//-azirines are readily transformed into 2,2-difluoroethylamines160168 by treatment with hydrogen fluoride/pyridine to give the products in moderate yield. Depending on the substituents, a-fluoro ketones are also formed in some cases.160169 Increased yields are obtained under similarly mild conditions with a weakened hydrogen fluoride system from 30% hydrogen fluoride/pyridine plus triethylamine.169... 

Iodofluorination and bromofluorination of alkenes are also effected in hydrogen fluoride systems by using bromine or iodine with an equivalent amount of silver(I) nitrate. In some cases, iodofluorination can also be achieved by iodine without adding the silver(I) salt, as illustrated in the following procedure. 

Fluorodcsulfurizations of thiols,311 phenyl thioglycosides,290 1,2-dithiolans.312 aromatic dithiocarboxylic acids,399 or orthothioesters313 in hydrogen fluoride systems with N-chloro-or N-bromosuccinimide as a coagcnt are facile methods for the generation of monofluorides, gem-difluorides, and trifluoromethyl derivatives, e.g. formation of 11,311 12,312 13,313 and... 

A variety of imidazole-,113148 thiazole- and isothiazoleearboxylic acids149 are stable enough to react with a sulfur tetrafluoride/hydrogen fluoride system modest to excellent yields of the corresponding (trifluoromethyl)imidazoles, -thiazoles and -isothiazoles can be obtained (Table 5). 

The reaction of 5-bromo-2-furoic acid with the sulfur tetrafluoridc/hydrogen fluoride system proceeds under mild conditions and results not only in conversion of the carboxylic acid group but also in addition of two fluorines and replacement or migration of bromine a 1 3.5 mixture of 2,2,5-trifluoro-5-(trifluoromethyl)-2.5-dihydrofuran (10) and 2-(bromodifluoromethyl)-2,5.5-trifluoro-2,5-dihydrofuran (11) is obtained.153... 

The same indicators have been used to compare the relative acidity of the three most used superacids. As shown on Figure 1.7, the half-protonation of indicator 3 necessitates 40, 25, and 8mol% SbF5, respectively, in triflic acid, fluorosulfuric acid, and hydrogen fluoride systems. These results show the supremacy of the HF-SbF5 system in which small concentrations of SbF5 induce a dramatic increase in acidity (see also Section 2.2.2.7). 

Latajka, Z., Bouteiller, Y., 1994, Application of Density Functional Methods for the Study of Hydrogen-Bonded Systems The Hydrogen Fluoride Dimer , J. Chem. Phys., 101, 9793. 

An approximate value for Xln can be deduced from Millen s original spectra. As described in Section 3.2 for the diethylether-hydrogen fluoride system, according to assignments made by Arnold and Millen 20) the main vj band is at 3405 cm-1. The two weaker bands at 3655 and 3225 cm-1 can be assigned to the summation tone (vj + v ) and to the difference tone (v3 — v ) respectively. It is important to remember in this respect (sect. 2.1) that while the frequency of a difference tone is the simple difference of the frequency of the two fundamentals ... 

Chlorofluorination of alkynes with the hydrogen fluoride/pyridine/A-chlorosaccharin system yields vicinal chlorofluoroalkenes with Markovnikov orientation for example, the formation of lA and... 

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