A description of the methods used to build a high quality, comprehensive reference library of electron-ionization mass spectra is presented. Emphasis is placed on the most challenging part of this project--the improvement of quality by expert evaluation. The methods employed for this task were developed over the course of a spectrum-by-spectrum review of a library containing well over 100,000 spectra. Although the effectiveness of this quality improvement task depended critically on the expertise of the evaluators, a number of guidelines are discussed which were found to be effective in performing this onerous and often subjective task. A number of specific examples of the particularly challenging task of spectrum editing are given.

C7H7+ ions with internal energies varying from ~0.5 to 6.5 eV above their ground-state energy have been prepared in an ion cyclotron resonance spectrometer (ICR) by charge-transfer-induced fragmentation of ethylbenzene, toluene, and norbornadiene. It is shown that in the case of (ethyl-a,a-¿2)benzene and toluene-a,a,a-if3 the abundance of unscrambled C6H5CD2+ ions produced by direct bond cleavage increases with energy to reach respectively 65 and 10% of the total C7(H,D)7+ population at 3 eV above the appearance energy. C7H5D2+ ions are also produced after the occurrence of ring expansion and contraction in the parent ion. However, because these ions produced from (ethyl-a,a-

7) F. P. Lossing. personal communication. The value of 156 kcai mol-' for AHf(CH&H=N+HZ) was derived from the AP of m l e 44 in the mass spectrum of (CH3)zCHNHz. We thank Dr. Lossing for his assistance in performing this measurement. (8) Values of 154 for AHf (CH3CH=N+HZ and CH3N+H=CHz reported by B. H. Solka and M. E. Russell, J. Phys. Chem., 78, 1268 (1974). (9) Values obtained from the proton affinities of NH3 and &CHzNH (207 and 220 kcal mol-l, respectively): I . The values for the heais of foimation of protonated vinyiamine and protonated vinyl alcohol were obtained by estimating the heats of formation and the proton affinities of the unprotonated species. The former were deduced using a group equivalent method and are in satisfactory agreement with the values found by calculations: L. Radom, W. J. Hehre, and J. A. Pople, J. Am. Chem. Soc., 93, 289 (1971). The proton affinities were taken to be the same as those measured for the corresponding saturated analogues, ethylamine and ethanol, respectively. G. Glockler, J. Phys. Chem., 61, 31 (1957).Abstract: The C7H7+ fragment ions formed in toluene, and the chlorinated, brominated, and iodinated toluenes, as well as the C,H6F+ fragment ions in fluorinated toluenes have been studied in an ion cyclotron resonance spectrometer. In all these systems, it is seen that there are two populations of C7H7+ ions, one of which reacts with the parent compound and other compounds, and one of which is totally unreactive. Because the reactive C7H7+ ions from the different precursors all undergo the same reactions and at the same rate as the C7H7+ ions in toluene and benzyl chloride, which have previously been shown to have the benzyl structure, this structure is assigned to the reactive C7H7+ populations; the unreactive C,H,+ ions are assigned the tropylium structure. The relative abundance of the unreactive tropylium ion is low ( < I 5%) in the chloro-, bromo-, and iodotoluenes, and decreases with decreasing energy of the ionizing electrons. This is interpreted to mean that the rearrangement to the seven-membered ring structure occurs after the fragment ion is formed. In toluene and fluorotoluenes, the unreactive component comprises 30-60% of the C7H6X' (X = H, F) ions, and decreases with increasing energy of the ionizing electrons, indicating that rearrangement occurs prior to the fragmentation of the parent ion. Rate constants for the hydride transfer to the benzyl ion from cyclopentane, cyclohexane, 3-methylpentane, and 3-methylhexane are reported. An estimate that LHr

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6505human platelet thromboxane synthetase inhibiting activity in vitro.32 In summary, the biological profile displayed by 2 is consistent with that of a TxA2 agonist. The agonist profile of 2, in contrast to the mixed activities exhibited by the carbon analogues of TxA2, indicates that the bicyclic acetal structure of TxA2, the assignment of which was based on indirect evidence,' plays a key role in the activity of the natural material. Furthermore, the reduced activity exhibited by 28 suggests that position of the acetal oxygens is important for activity. A detailed description of these results will be the subject of a forthcoming publication. (32) Needleman, P.; Moncada, S.; Bunting, S.; Vane, J. R.; Hamberg, M.; Samuelsson, B. Nature (London) 1976, 261, 558.For many years, the organometallic chemistry of the lanthanide elements has centered primarily on the 3+ oxidation states2 During the past few years, we have investigated the low valent chemistry of the lanthanide elements in an effort to develop a more extensive chemistry for these metals than is found for their trivalent and recently, interest in this area has increased As part of our research program, we have used the metal vaporization technique1z-18 to examine the reactivity