Isotope spectra

Kr isotopic data derived from either stepwise or total sample analysis represent generally mixtures of several components, but equations (4) and (5) require the identification of the cosmic ray produced spallation component. As long as spallation Kr is the major component, the analysis of isotopic spectra is reasonably... 

Nuclear magnetic resonance spearoscopy—Congresses. 2. Carbon—Isotopes—Spectra—Congresses. 3. Polymers and polymerization—Spectra—Congresses. 

Isotope spectra. Deuterium (D) is an isotope of hydrogen (H) with a mass about twice that of hydrogen electronic properties of H2 and D2,... 

Isotope substitution will have four principal effects of interest here but, apart from these, isotopic spectra should have the same total intensities (zeroth-order sum formulae are not mass dependent). Specifically, one would expect that... 

Isotope spectra. Rotovibrational spectra of deuterium, and of deuterium-rare gas mixtures, have also been recorded over a wide range of temperatures and densities [342]. The differences between the H2-X and D2-X spectra (with X = H2 or D2, respectively, or a rare gas atom) are much like what has been seen above for the rototranslational spectra. 

Obviously, there is a dUenuna here. Without doubt, the EFFF cau be used to fit complicated isotopic spectra with remarkable accuracy, and yet the force constants it uses appear to be at variance with the true force constants. If the F matrix of the EFFF is only a gross approximation of the true (viz. GQVFF) F matrix, then why does the EFFF work so well ... 

Burdett etal " showed that, to fit isotopic spectra accurately, the EFFF needed two corrections a factor that takes into account the particular isotope of CO used and a further correction to ameliorate the effects of using anharmonic frequencies. The effect of the neglect of M-C/C-O conphng was originally explained by Miller in terms of an x-factor, which depended on the particular CO isotope being nsed. Anharmonicity is accounted for by another correction that could be determined from the all- C 0 molecnle and the reduced mass ratio. 

The IR spectrum of complexes such as these will therefore show a single band (the mode). If Raman data are unavailable, and the supposedly IR-inactive bands are not observed as weak features, reasonably accurate force constants can be obtained from equation (18). More usually, asymmetry in the ligand or distortion of the force field due to solvent-CO interactions makes both the Raman-active bands very slightly allowed in the IR. Indeed, trans-[ i-PrO)3P]2Mo(CO)4 shows pronounced effects due to the irregular packing of the phosphite ligands. Not only are the a g and big bands readily observable, but the band is rather broader than is usually found. In the absence of such data, isotopic spectra will provide enough information to resolve the problem. The monosnbstitnted molecnle is present at 4.4% (natural abundance). The force fields of this type of molecule invariably support Cotton s approximation... 

Amines - Spectra. 2. Alkaloids — Spectra. 3. Carbon - Isotopes — Spectra. 4. Nuclear magnetic resonance spectroscopy. I. Hindenlang, David M., joint author, n. Title. 

In combination with mass spectrometry isotope selective spectroscopy becomes possible, even if the spectral lines of the different isotope spectra... 

N-enriched samples of compound 35 in combination with the natural isotope spectrum enabled the unambiguous assignment of the structure. 

This wonld, assnming a particnlar C bond angle to provide appropriate valnes of A and B, enable an approximate force field to be calcnlated. That said, accnrate force constants can only be obtained from the isotopic spectrum. 

The a[ band is IR-inactive but is often present as a very weak feature. If Raman spectra are not available to confirm the location of this absorption, the force field can be determined from the isotopic spectrum. Naturally abundant would provide for 3.3% of the molecules being isotopically substituted and this would give rise to the usual satellite bands. 

The model spectra show that the only region where there is any significant variation in the profile is in the C-H out-of-plane bending region around 880 cm exactly the region where the spectra of the carbons show variations. The INS spectrum of a single HI or H2 or H3 imit can be examined by calculating an isotopic spectrum where the... 

Nucleosynthesis studies. For nueleosynthesis studies, a neutron irradiation, much the same as the ones for " Ar- Ar or 1-Xe, is done. But the reaction of interest is neutron-introdueed fission of U. As noted in Table 2, this produces a different isotopic spectrum than decay. Since there are four isotopes affected, and only two spectra, it... 

First, why are there these strange doublets in the IR spectrum for HCl Well, the reason is quite trivial two natural chlorine isotopes Cl and Cl, which are always present in the natural specimen (with proportion 3 1). The H Cl molecule rotates (as well as oscillates) differently than the H Q because of the reduced mass difference [see Eq. (6.29)]. This difference of p. is very small, since what decides in /r is the small mass of the proton. Thus, these two molecules will correspond to two spectra that are similar, but shifted a bit with respect to one another on the frequency axis, the heavier isotope spectrum corresponding to a bit lower frequency. 

It is also usually possible to remove all the couplings from a particular isotope, e.g. H, provided that one only wishes to observe the spectrum from another isotope, e.g. Either the decoupling frequency is noise-modulated to cover the relevant range of chemical shifts, or else the same decoupling is achieved more efficiently, and with less heating of the sample, by using a carefiilly designed, continuous sequence of... 

Figure Bl.25.9(a) shows the positive SIMS spectrum of a silica-supported zirconium oxide catalyst precursor, freshly prepared by a condensation reaction between zirconium ethoxide and the hydroxyl groups of the support [17]. Note the simultaneous occurrence of single ions (Ff, Si, Zr and molecular ions (SiO, SiOFf, ZrO, ZrOFf, ZrtK. Also, the isotope pattern of zirconium is clearly visible. Isotopes are important in the identification of peaks, because all peak intensity ratios must agree with the natural abundance. In addition to the peaks expected from zirconia on silica mounted on an indium foil, the spectrum in figure Bl. 25.9(a)...

Searches for the element on earth have been fruitless, and it now appears that promethium is completely missing from the earth s crust. Promethium, however, has been identified in the spectrum of the star HR465 in Andromeda. This element is being formed recently near the star s surface, for no known isotope of promethium has a half-life longer than 17.7 years. Seventeen isotopes of promethium, with atomic masses from 134 to 155 are now known. Promethium-147, with a half-life of 2.6 years, is the most generally useful. Promethium-145 is the longest lived, and has a specific activity of 940 Ci/g. 

The study of the infrared spectrum of thiazole under various physical states (solid, liquid, vapor, in solution) by Sbrana et al. (202) and a similar study, extended to isotopically labeled molecules, by Davidovics et al. (203, 204), gave the symmetry properties of the main vibrations of the thiazole molecule. More recently, the calculation of the normal modes of vibration of the molecule defined a force field for it and confirmed quantitatively the preceeding assignments (205, 206). 

Out-of-Plane Vibrations, yCH and yCD. In accordance with all the proposed assignments (201-203), the bands at 797 and 716 cm correspond to yCH vibrators, which is confirmed by the C-type structure observed for these frequencies in the vapor-phase spectrum of thiazoie (Fig. 1-9). On the contrary, the assignments proposed for the third yCH mode are contradictory. According to Chouteau et al. (201), this vibration is located at 723 cm whereas Sbrana et al. (202) prefer the band at S49cm and Davidovics et al. (203) the peak at 877 cm This last assignment is the most compatible with the whole set of spectra for the thiazole derivatives (203) and is confirmed by the normal vibration mode calculations (205) (Table 1-25). The order of decreasing yCH frequencies, established by the study of isotopic and substituted thiazole derivatives, is (203) yC(4)H > 70(2)13 > yC(5)H. Both the 2- and 4-positions, which seem equivalent for the vCH modes, are quite different for the yCH out-of-plane vibrations, a fact related to the influence observed for the... 

There is a small peak one mass unit higher than M m the mass spectrum of ben zene What is the origin of this peak d What we see m Figure 13 40 as a single mass spectrum is actually a superposition of the spectra of three isotopically distinct benzenes Most of the benzene molecules contain only and H and have a molecular mass of 78 Smaller proportions of benzene molecules contain m place of one of the atoms or m place of one of the protons Both these species have a molecular mass of 79... 

Not only the molecular ion peak but all the peaks m the mass spectrum of benzene are accompanied by a smaller peak one mass unit higher Indeed because all organic com pounds contain carbon and most contain hydrogen similar isotopic clusters will appear m the mass spectra of all organic compounds... 

IS the only phosphorus isotope present at natural abundance and has a nuclear spin of The H NMR spectrum of tnmethyl phosphite (CH30)3P exhibits a doublet for the methyl protons with a splitting of 12 Hz... 

A more recent experimental technique employs C as the isotopic label Instead of locating the position of a label by a laborious degradation procedure the NMR spectrum of the natural product is recorded The signals for the carbons that are enriched m are far more intense than those corresponding to carbons m which IS present only at the natural abundance level... 

Mass spectrum of a carbon compound with (a) and without (b) the C isotopes. 

Many elements have two or more isotopes, and the presence of these correspondingly gives a spectrum having many peaks. On the other hand, the patterns of these isotopes in a mass spectrum often prove invaluable in identifying which elements are present, with the patterns serving as fingerprints for certain elements. 

In a process similar to that described in the previous item, the stored data can be used to identify not just a series of compounds but specific ones. For example, any compound containing a chlorine atom is obvious from its mass spectrum, since natural chlorine occurs as two isotopes, Cl and Cl, in a ratio of. 3 1. Thus its mass spectrum will have two molecular ions separated by two mass units (35 -i- 2 = 37) in an abundance ratio of 3 1. It becomes a trivial exercise for the computer to print out only those scans in which two ions are found separated by two mass units in the abundance ratio of 3 1 (Figure 36.10). This selection of only certain ion masses is called selected ion recording (SIR) or, sometimes, selected ion monitoring (SIM, an unfortunate... 

Naturally occurring isotopes of any element are present in unequal amounts. For example, chlorine exists in two isotopic forms, one with 17 protons and 18 neutrons ( Cl) and the other with 17 protons and 20 neutrons ( Cl). The isotopes are not radioactive, and they occur, respectively, in a ratio of nearly 3 1. In a mass spectrum, any compound containing one chlorine atom will have two different molecular masses (m/z values). For example, methyl chloride (CH3CI) has masses of 15 (for the CH3) plus 35 (total = 50) for one isotope of chlorine and 15 plus 37 (total = 52) for the other isotope. Since the isotopes occur in the ratio of 3 1, molecular ions of methyl chloride will show two molecular-mass peaks at m/z values of 50 and 52, with the heights of the peaks in the ratio of 3 1 (Figure 46.4). 

A diagrammatic illustration of the effect of an isotope pattern on a mass spectrum. The two naturally occurring isotopes of chlorine combine with a methyl group to give methyl chloride. Statistically, because their abundance ratio is 3 1, three Cl isotope atoms combine for each Cl atom. Thus, the ratio of the molecular ion peaks at m/z 50, 52 found for methyl chloride in its mass spectrum will also be in the ratio of 3 1. If nothing had been known about the structure of this compound, the appearance in its mass spectrum of two peaks at m/z 50, 52 (two mass units apart) in a ratio of 3 1 would immediately identify the compound as containing chlorine. 

Isotopes of an element are formed by the protons in its nucleus combining with various numbers of neutrons. Most natural isotopes are not radioactive, and the approximate pattern of peaks they give in a mass spectrum can be used to identify the presence of many elements. The ratio of abundances of isotopes for any one element, when measured accurately, can be used for a variety of analytical purposes, such as dating geological samples or gaining insights into chemical reaction mechanisms. 

In a mass spectrum, the ratios of isotopes give a pattern of isotopic peaks that is characteristic of a given element. For example, the mass spectrum of any corn ound containin carbon, hydrogen, nitrogen, and oxygen will show patterns of peaks due to the, 7C, 7N, gO, gO, and... 

Special isotope ratio mass spectrometers are needed to measure the small variations, which are too small to be read off from a spectrum obtained on a routine mass spectrometer. Ratios of isotopes measured very accurately (usually as 0/00, i.e., as parts per 1000 [mil] rather than parts per 100 [percent]) give information on, for example, reaction mechanisms, dating of historic samples, or testing for drugs in metabolic systems. Such uses are illustrated in the main text. 

Isotope ratios are very useful for (a) identifying elements from their pattern of isotopes in a spectrum obtained on an ordinary mass spectrometer or (b) obtaining detailed information after accurate measurement of isotope ratios from special isotope ratio instruments. 

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