Isotopic carbon monoxides

In Section V.D a theory based on the results of Section III.D is proposed in order to explain the anomalous relative vapor pressures of isotopic carbon monoxides and nitric oxides. 

TABLE V-5. Relative Vapor Pressures of Isotopic Carbon Monoxides... 

Because of the present unsatisfactory status of our knowledge of intermolecular forces, we are unable to evaluate the parameter d and consequently, we cannot evaluate R T). The theory given in this section (V-31) then only allows us to predict that the values of the ratio R(T) as calculated from the experimental vapor pressures of three isotopes will prove to be independent of temperature. This prediction is verified in the case of isotopic carbon monoxides and nitric oxides as can be seen from Tables V-5 — V-10. 

Table V-6 shows that, in the case of the isotopic carbon monoxides C1301. and...

The interstitial air trapped during this process preserves a largely unaltered record of the composition of past atmospheres on time scales as short as decades and as long as several hundred thousand years. Such records have provided critical information about past variations in carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), carbon monoxide (CO), and the isotopic composition of some of these trace species. In addition, studies of the major elements of air nitrogen, oxygen, and argon, and their isotopic composition, have contributed... 

Yeom and Frei [96] showed that irradiation at 266 nm of TS-1 loaded with CO and CH3OH gas at 173 K gave methyl formate as the main product. The photoreaction was monitored in situ by FT-IR spectroscopy and was attributed to reduction of CO at LMCT-excited framework Ti centers (see Sect. 3.2) under concurrent oxidation of methanol. Infrared product analysis based on experiments with isotopically labeled molecules revealed that carbon monoxide is incorporated into the ester as a carbonyl moiety. The authors proposed that CO is photoreduced by transient Ti + to HCO radical in the primary redox step. This finding opens up the possibility for synthetic chemistry of carbon monoxide in transition metal materials by photoactivation of framework metal centers. 

Exchange reactions between bulk and adsorbed substances can be studied by on-line mass spectroscopy and isotope labeling. In this section the results on the interaction of methanol and carbon monoxide in solution with adsorbed methanol and carbon monoxide on platinum are reported [72], A flow cell for on-line MS measurements (Fig. 1.2) was used. 13C-labeled methanol was absorbed until the Pt surface became saturated. After solution exchange with base electrolyte a potential scan was applied. Parallel to the current-potential curve the mass intensity-potential for 13C02 was monitored. Both curves are given in Fig. 3.1a,b. A second scan was always taken to check the absence of bulk substances. 

Example the molecular ions of nitrogen, N2, carbon monoxide, CO, and ethene, C2H4, have the same nominal mass of 28 u, i.e., they are so-called iso-baric ions. The isotopic masses of the most abundant isotopes of hydrogen, carbon, nitrogen and oxygen are 1.007825 u, 12.000000 u, 14.003070 u and 15.994915 u, respectively. Using these values, the calculated ionic masses are 28.00559 u for Nz"" , 27.99437 u for CQ-", and 28.03075 u for CjH/. This means they differ by some millimass units" (mmu) from each other, and none of these isobaric ions has precisely 28.00000 u (Chap. 3.3.4 and Chap. 6.9.6). 

Prior to 1970 our understanding of the bonding of diatomic molecules to surfaces, and in many cases the type of adsorption (i.e., molecular or dissociative) was almost entirely dependent on indirect experimental evidence. By this we mean that deductions were made on the basis of data obtained from monitoring the gas phase whether in the context of kinetic studies based on gas uptake or flash desorption, mass spectrometry, or isotopic exchange. The exception was the important information that had accrued from infrared studies of mainly adsorbed carbon monoxide, a molecule that lent itself very well to this approach owing to its comparatively large extinction coefficient. 

For the determination of energy factored parameters, full isotopic substitution is bound to be useless. This is because, in the energy factored approximation, such substitution is predicted to multiply each frequency by a factor of [fij/j, ), where fx is the reduced mass mcfnol[nic+ o] of carbon monoxide, independent of the values of the parameters concerned. However, a great deal of information can be obtained from the spectra of partially substituted species, for which the Teller-Redlich product rule takes the particularly simple form... 

The deoxygenation of tetrahydrofuran (THE, 90), which yields ethylene and carbon monoxide, is an interesting case. While this and other deoxygenations might be expected to proceed through an yild intermediate and a biradical as shown in Eq. 50, calculations (MP2/6-31G ) indicate that neither ylid 91 nor biradical 92 is an intermediate in this reaction. These calculations reveal a concerted removal of oxygen that proceeds to carbon monoxide and two molecules of ethylene without barrier. Experimental evidence that 91 is not an intermediate is provided by the fact that reaction of carbon with a mixture of 90 and 90-d generates ethylene and ethyl-ene-t/g in a 2.7 1 ratio.This secondary isotope effect of 1.13 (per D) would not be expected if 91 (or 92) were an intermediate. 

Isotope labeling studies furnished a convincing body of evidence to indicate that even in the presence of carbon monoxide, only carbon dioxide is converted to methanol [Eq. (3.2)].394-398 In a review article371 Chinchen considers this as the dominant route, with CO merely serving as a source of C02 via the water-gas shift reaction. 

Fig. 14. EPR spectra of carbon monoxide oxidoreductase from C. thermoaceticum, treated with CO plus coenzyme A. Solid lines are experimental spectra, dashed lines are computer simulations, with gx= gf = 2.074, g2 = 2.028. Substitutions with 61Ni and 57Fe were made by growth of the organism on the appropriate isotopes, (a) Effect of substitution with 6lNi. Simulation assumes AM = 3 MHz, A, = 20 MHz. (b, p. 328) Effects of substitution with 5,Fe and l3C. The simulation of the 57Fe spectrum assumes one iron atom with Ah = 40 MHz, A = 60 MHz, and two iron atoms with A, = 20 MHz, A = 30 MHz. The simulation of the l3C spectrum assumes An = 26 MHz, A = 13 MHz. Spectra provided by courtesy of Dr. S. G, Ragsdale.

The stages of isotopic exchange mechanism (357), if we do not distinguish between isotopes H and D, coincide with the forward and reverse directions of stage 1 of mechanism (343) of carbon monoxide conversion. The reactions of isotopic exchange corresponding to stage 2 or this mechanism... 

On Fe304 the rates of isotopic exchange reactions (356), (366), and (367) are close to the rate of carbon monoxide conversion, as should be expected from mechanism (343). 

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