H2 + molecular ion

A recent success in the detection of H species has been that of the molecular ion H3+. All of the models of ion-molecule chemistry in hydrogen-dominated regions are controlled by reactions of H3+ but until recently the H2+ molecular ion had not been detected. However, the modes of vibration of H3"1" provide for an allowed IR transition at 3.668 pin used for its detection. These ro-vibrational transitions have now been observed in a number of places, including the interstellar medium and in the aurorae of Jupiter. Not all astronomical detection and identification problems have been solved, however, and the most annoying and compelling of these is the problem of diffuse interstellar bands. 

In still other experiments, selective excitation of the H2+ molecular ion in collisions with rare gases at 0.1 to 10 keV was studied by determining the polarizations of the Balmer-a and Balmer-/ lines resulting from the dissociative collision.293... 

We are ready now to make predictions about the stability of H2, H2, He2, and He2 (see Figure 10.24). The H2 molecular ion has only one electron, in the o-ij orbital. Since a covalent bond consists of two electrons in a bonding molecular orbital, has only half of one bond, or a bond order of Thus, we predict that the H2 molecule may be a stable species. The electron configuration of is written as (o-i ) ... 

Even if no integral parametrizations are introduced and the HF equations are all correctly solved, the method eventually turns out to be theoretically incomplete. Despite the correct treatment of electronic exchange (X) within Hartree-Fock theory, electronic correlation (C) is totally missing. This is easily shown for the case of the H2 molecule in which we use the bonding solution of the H2 molecular ion ( + = cr from Equation (2.15)) to build up an antisymmetrized molecular wave function. This means that we put both electrons (ri and rz) of the H2 molecule into the same ip+ orbital, and Pauli s principle is obeyed by means of the ct/ spinors. Neglecting orbital overlap and any pre-factors, for simplicity, the so-called Hund-Mulliken [124] (another name... 

Bond orders need not be whole numbers half-integral bond orders of 2, 2. and so forth are also possible. For example, the H2 molecular ion, which is formed in mass spectrometers, has the configuration (cti ) and a bond order of 2(1 0) = 2-... 

This time the normalization constant has —512 in the denominator and so 2Af2o > 1. confirming that this term gives an overall negative contribution to the bond formation energy for an H2+ molecular ion with the electron in the antibonding orbital. 

High mass resolution techniques are used to separate peaks at the same nominal mass by the very small mass differences between them. As an example, a combination of Si and H to form the molecular ion Si H , severely degrades the detection limit of phosphorous ( P) in a silicon sample. The exact mass of phosphorous ( P) is 31.9738 amu while the real masses of the interfering Si H and Si H2 molecules are 31.9816 amu and 31.9921 amu, respectively. Figure 8 shows a mass... 

Neutral Loss Only a limited number of neutral fragments of low mass which are eliminated in decompositions of molecular ions. Examples are H, H2, CH3 and OH. Therefore, the presence of a major ion below the molecular ion at an improbable interval (eg, loss of 4 to 14, 21 to 25 amu) will indicate that the latter is not the molecular ion Postulation of Molecular Structures The. postulation of the structure of an unknown molecule is based on several major kinds of general structural information available in the mass spectmm. McLafferty (Ref 63) suggests the following systematic approach ... 

Table 2 The molecular ions observed during acetone condensation reactions under H2 and D2.

In quantum mechanics, as we have already seen, one can approximately describe the hydrogen molecular ion as consisting of Ha+ and Hb, or Hb+ and Ha. Some combination of wave functions representing these two configurations is needed as an approximation of the actual state of affairs. The state of H2+ can then be thought of as a resonance hybrid of the two. 

Example In the electron ionization mass spectmm of a hydrocarbon, the molecular ion peak and the base peak of the spectrum correspond to the same ionic species at m/z 16 (Fig. 1.2). The fragment ion peaks at m/z 12-15 are spaced at 1 u distance. Obviously, the molecular ion, M" , fragments by loss of H which is the only possibility to explain the peak at m/z 15 by loss of a neutral of 1 u mass. Accordingly, the peaks at lower m/z might arise from loss of a H2 molecule (2 u) and so forth. It does not take an expert to recognize that this spectrum belongs to methane, CH4, showing its molecular ion peak at m/z 16 because the atomic mass... 

The peaks in the m/z 50-57 range of the 1-butene El spectrum could be misinterpreted as a complex isotopic pattern if no formula were available on the plot (Fig. 3.8). However, there is no element having a comparable isotopic pattern and in addition, all elements exhibiting broad isotopic distributions have much higher mass. Instead, the 1-butene molecular ion undergoes H, H2 and multiple H2 losses. The m/z 57 peak, of course, results from In a similar fashion the peaks at m/z 39 and 41 appear to represent the isotopic distribution of iridium, but this is impossible due to the mass of iridium (cf. Appendix). However, these peaks originate from the formation of an allyl cation, CsHs, m/z 41, which fragments further by loss of H2 to form the CsHs" ion, m/z 39 (Chap. 6.2.4). 

C) mixture H2. Nearly all the ions are from the protein, buffer and solvent background except for the ions at m/z 145.8 and mjz 155.7. These two ions are protonated molecular ions for compounds with MWs of 145 Da and 155 Da that bind to MMP-1. Reprinted from reference [1] with permission from Elsevier Science. 

Innorta et al. (206), in a study of carbonyl complexes containing organic rings, have rationalized the loss of small stable molecules, rather than radicals, in terms of the activation energies for fragmentation pathways possible from the molecular ion. Thus loss of H2 after CO has been removed can become common if it results in aromatization of a ring. 

In fact, the measured dissociation energies of appropriate examples of homo-nuclear diatomic molecules and molecular ions are H, 2.648 e.v. H2, 4.476 e.v. He, 3.1 e.v. He2, only slight attraction in the ground electronic state (binding of van der Waals type, at internuclear separations large compared with typical chemical binding energies.)... 

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