Gas-phase measurements

Bohme D K and Raksit A B 1984 Gas phase measurements of the influence of stepwise soivation on the kinetics of nucieophiiic dispiacement reactions with CHjCi and CHjBr at room temperature J. Am. Ghem. Soc. 106 3447-52... 

Unlike in the case of the gas-phase measurements, no tunneling has been detected in the IR spectra of the malonaldehyde molecule in the noble matrices at 15-30K [Firth et al. 1989], The lack of tunneling is caused by detuning of the potential as a result of weak antisymmetric coupling to the environment. 

Reliable information on the thermodynamic stability of group 13/15 adducts is usually obtained by gas phase measurements. However, due to the lability of stibine and bismuthine adducts in the gas phase toward dissociation, temperature-dependent H-NMR studies are also useful for the determination of their dissociation enthalpies in solution [41b], We focussed on analogously substituted adducts t-BusAl—E(f-Pr)3 (E = P 9, As 10, Sb 11, Bi 12) since they have been fully characterized by single crystal X-ray diffraction, allowing comparisons of their thermodynamic stability in solution with structural trends as found in their solid state structures. 

Acknowledgment The authors express their thanks for financial support from the DFG grants JE 267/2-1 and 436 RUS 113/788/0-1 and the Program N 1 of the Russian Academy of Sciences. Jutta Krohnert is acknowledged for propene gas phase measurements. 

BC13, BBr3 and BI3. B-N distances obtained from single crystal X-ray diffraction studies as well as gas phase measurements yielded only small variations (X-ray 158.5-161.Opm gas phase 165.2-167.4pm),38 again demonstrating that the thermodynamic stability of group 13/15 adducts not necessarily correlates with their M-E bond length. 

The linear structure of 93 was derived from experiments with labeled precursor molecules and by correlation of vibrational frequencies calculated from estimated force constants with the recorded IR absorptions. The three fundamentals were observed as well as the UV/VIS spectrum,131 which was resolved and analyzed by gas phase measurements.132 The predicted triplet ground state was confirmed by recording the ESR spectrum of 93 isolated in various matrices.131... 

From gas phase measurements CO is known to prefer top sites on all three low index faces, with the CO molecule perpendicular to the surface and bonded through the carbon end of the molecule except at high coverages (27). It is likely that HCOOH and COOH are adsorbed in a similar way. It is not likely that they could "enter the "troughs , which seems to be possible for anions. For Pt(100) on the other hand, upon sweep reversal and gradual oxide reduction, anions are immediately adsorbed on that "flat" surface. They block adsorption of HCOOH. Adsorption of anions decreases as potential becomes more negative. The oxidation of HCOOH commences and the rate increases as at more negative potentials, i.e. at lower overpotential. A competition between anions and HCOOH adsorption explains this apparently anomalous behaviour. The explanation of the "anomalous behaviour of the Pt(110) surface can be also found in the data for stepped surface vicinal to the (100) and (110) orientations. 

Gas phase measurements of the formation of protonated species can provide similar thermodynamic data. 

The gas is another phase which is detectable by microphonic photoacoustic method. The focused beam (at Beam Line 15A (PF)) at wavelength of 1.56 A was used for this experiment. Photoacoustic cell for gas phase measurement is shown in Fig. 9. The optical path was 10cm (x6mm O with beryllium windows (18mm d>, 0.5mm... 

The capacity of inositol orthoformate derivatives 124 and 125 for binding to alkali metal ions was studied by electrospray ionization mass spectrometry (ESI-MS) gas-phase measurements <2001JOC8629>. The [5.5.5]-iono-phore 125 n = 3) possessed the highest Li /Na selectivity and the best affinity for Li. The results obtained proved to be in agreement with the size-fit concept. Other factors which influence the complexation are the orientation of the oxygen atoms, which are able to bind to metal, the basicity, and the polarizability of the heteroatoms around the perimeter of the binding cavity. 

When it is important to control the water activity in a reactor, a water activity sensor is quite useful. The sensor should ideally measure the water activity in the liquid reaction medium. However, the sensors available are designed for gas phase measurements, and, provided there is effective enough equilibration between the liquid and gaseous phases, they can be used to control the water activity in the reactor. If the measured water activity is above the set point, drying is initiated, for example, by passing dry air through the reactor. On the other hand, if the water activity is too low, water can be added, either as liquid water or as humid air. Automatically controlled systems of this kind have been successfully used to monitor and control enzymatic reactions in organic media [13, 14]. 

The over-riding significance of gas-phase measurements is that in the gaseous state the contributions of electrostatic and even of London dispersion forces can be reasonably estimated this allows of more significant comparison with the observed AH values. At this point it may be appropriate to emphasize the variation in AH with the medium in which it is observed. If the free energy increase (AG) attending the dissociation of the dimer were to vary with the dielectric constant (D) according to... 

J. S. Rowltnson (Manchester) I am very glad that Dr. Davies has stressed the importance of gas-phase measurements of the second virial coefficient as a measure of the strength of the bonds between dimers. The second virial coefficient is undoubtedly the simplest physical property which refers solely to the interaction of molecules in pairs. In particular, I would point out that the second virial coefficients of methanol give a heat of association of about 4-5 fecal/mole. The measurements would not, I think, be consistent with a heat as high as the 9 kcal reported by Dr. E. D. Becker for the formation of dimer in carbon tetrachloride. Moreover, the effect of the medium would be expected to bring a lowering of the gas-phase heat, as Dr. Davies has shown. 

G. C. Pimentel (Berkeley) In rebuttal of Dr. Rowlinson s remark that the gas-phase measurements should be given more weight than other types of measurements, we must remember that the measurements made in different phases may not give the same answer, nor may they refer to the same species. This is suggested by the unexpectedly high values of ZlH of H-bond formation shown by Dr. Davies for solid amides. While this AH may include certain interactions not present in the gas phase, and while it may be more difficult to interpret, nevertheless it is the AH appropriate to the solid structure and one cannot determine this value by gas-phase studies. Similarly, in the liquid phase there may be different processes or species than in the gas phase. Hence one cannot discard a measurement of AH of dimerization applicable to solution phase solely because it disagrees with measurements applicable to the gas phase. 

The Br0nsted gas-phase acidity will be related to the proton affinity of the conjugate base. However, this gives us no estimate of the relative acidity of nonprotonk (Lewis) acids, (f the electron is the bask analogue of the acidk proton, then electron affinities should provide an inherent gas-phase measure of acidity that parallels proton affinities for bases.— That they have not been more frequently used in this connection is... 

The conclusion that should be drawn from this discussion is that there are two kinds of acidity that must not be confused (1) an intrinsic acidity, which is best approximated by gas-phase measurements and which reflects the properties of the ions and molecules in isolation, and (2) a practical liquid-phase acidity in which solvation effects may play the dominant role. In interpretation of structure-reactivity relationships, the liquid-phase acidity will probably be misleading unless the structures being compared are very similar for thinking about chemical behavior in solution, however, the liquid-phase acidities are clearly the important ones. 

The oxygen and sulfur bases are weaker than the nitrogen bases, and accurate solution basicities are not available. Arnett s heat of protonation studies indicate that the order of decreasing basicity is ROR > ROH > HaO,98 a result that is in agreement with gas-phase measurements.99 Hydrogen sulfide in the gas phase has basicity comparable to that of water (Table 3.18), and substitution of H by alkyl produces stronger gas-phase bases just as does similar substitution on oxygen. 

The determination of the strength of the Lewis acids MF , has been carried out in various solvents using the conventional methods. Numerous techniques have been applied conductivity measurements, cryoscopy, aromatic hydrocarbon extraction,53,84 solubility measurements,85-87 kinetic parameters determinations,52,88,89 electroanalytical techniques (hydrogen electrode),90-93 quinones systems as pH indicators,94-97 or other electrochemical systems,98 99 IR,100,101 and acidity function (//,) determinations with UV-visible spectroscopy,8 9 14 19 102-105 or with NMR spectros-copy.20-22,44-46,106-108 Gas-phase measurements are also available.109-111 Comparison of the results obtained by different methods shows large discrepancies (Table 1.2). 

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