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Figure 5. Bond moments and assumed configuration for the dichloroacetic acid-pyridine complex.
of 2.4 D for ethyl dichloroacetate; the lit’erature value is 2.63 Dg). Complex formation leads to increased polarity in the 0-H----N bond. The dipole moment of
the complex (5.45 D) is reproduced if the moment of the (0-H----N) group is increased from 3.7 to 5.1 I). With an OH bond length of 1.02 A in the complex,2this leads to an effective charge of 2.8 X 10-’0 esu at, the 0 and H centers. I n the free acid the effective charge at the 0 and H centers (separation, 0.96 A) is 1.57 X lO-’O esu. Thus complex formation nearly doubles the ‘‘ionicity’’ of the OH bond. This result is not unreasonable in view of the approximations involved in the group moment calculations and the neglect of polarizability effects. They suffice, however, to establish the qualitative conclusion that the proton remains predominantly attached to the oxygen atom rather than to the nitrogen atom. (9) A. L. hfcClellan, “Tables of Experimental Dipole Moments,” W. H. Freeman and Co., San Francisco, Calif., 1963.
COMMUNICATIONS T O T H E EDITOR Charge Distribution in Some Alkanes and Their Mass Spectra
Si?.: I n the past 15 years several papers’-5 have appeared discussing the possibility of predicting the fragmentation pattern of organic ion-molecules on the basis of a proposal by Lennard-Jones and Hall,’ who supposed the probability of fragmentation to be larger for those bonds where the density of the net positive charge is larger. Some authors3-j have pointed out that such a proposal appears to be inconsistent with the experimental results, on the basis of a comparison of the mass spectra of n-alkanes (which show a pronounced maximum at Ca, sometimes C4, fragments) with their calculated net positive charge distributions (which have a maximum at the center of the molecule). Surprisingly enough, these authors do not seem to take into account what seems to us a primary question, Le., to what extent does the distribution of the ions collected by the mass spectrometer reproduce that originating from the first fragmentation of the parent ion, thus accepting a sort of unjustified identification between mass spectrum and fragmentation pattern. Xore recently, Lorquet5 makes the still more surprising statement that the Lennard-Jones and Hall hypothesis (which, he says, does not hold for n-alkanes, due to a “too evenly distributed charge”) is experimentally verified in the mass spectra of branched alkanes. He supports his statement by the mass spectra of five hydrocarbons. The Journal of Phgsical Chemislry
It should be pointed out, however, that all the alkanes considered by this last author have such a formula as to give C3 or Cq fragments on fission of the most highly charged bond, i.e., those fragments which are in any case the most abundant. I n fact, an inspection of the mass spectra of two branched alkanes of larger sizeeand of their net positive charge distributions (Figure l),calculated by us for comparison (by the same method as Lorquet’s), shows the following facts: (1) the only feature which clearly distinguishes these spectra from those of the n-alkanes with the same number of C atoms is the higher abundance of the ions c n - 6 and C n-8; (2) fragments which cannot originate by a single fragmentation (ie., those ranging from C7 to Clz for the first compound and from C9 to CIS for the second one) are roughly as abundant as in the spectra of the normal alkanes with the same number of C atoms. From these observations we must conclude that the shape of the spectra is largely determined by refragmentation phenomena; as a consequence, the well-known “weakness” of the tertiary bonds is observable only through
(1) J. Lennard-Jones and G. G. Hall, Trans. Faraday Soc., 48, 581 (1952). (2) R. Thompson, Conference on Applied Mass Spectrometry, Institute of Petroleum, London, 1953, p 154. (3) N. D. Coggeshall, J. Chem. Phgs., 30, 593 (1959). (4) K.Fueki and K. Hirota, N i p p o n Kagalcu Zasshi, 81, 212 (1960). (5) J. C. Lorquet, Mol. Phys., 9, 101 (1965). (6) The spectra were obtained from those of American Petroleum Institute Project 44, Carnegie Institute of Technology, Pittsburgh, Pa., by normaliring to 100 the sum of the abundances of all the ions.
COMMUNICATIONS TO THE EDITOR
463 Reply to ‘‘Charge Distribution in Some Alkanes and Their Mass Spectra”
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Figure 1. (a) Histograms representing mass spectra. T h e abscissa gives t h e number of C atoms of the fragment. (b) Schematic representation of the percentage distribution of the net positive charge on C-C bonds.
the very small peaks corresponding to the large (secondary) Eragments. Owing to this situation, it is not admissible to test any theoretical prediction by means of so simple and direct a comparison with mass spectra as done by Lorquet and others. On the other hand, an accurate interpretation of the mass spectra, which takes into account the contribution of refragmentation, is not a t all a simple problem; therefore we must more generally conclude that o d i n a r y mass spectrometry is an experimental technique not suitable for the measurement of fragmentation patterns.’ Some information may be obtained by several other means. Preliminary results obtained by us by measurements of the products of liquid radiolysis of n-alkanes seem, for instance, to indicate a substantially flat fragmentation pattern, except for the first bond, which appears to have a lower fragmentation probability. (7) We are very glad to ascertain that a substantial agreement on the interpretation of the mass spectra has been reached (see the Lorquet and Hirota communications in this issue). It is our opinion that such an agreement was not at all apparent in the papers available until a few months ago. We feel that our ideas were perhaps not clearly enough explained on two topics and they must be briefly clarified. (a) When we state that refragmentation largely determined the ordinary mass spectra of the alkanes, we mean also those of low or medium molecular weight. See, for instance, the mass spectra of 3-methylpentane and of 4-methylheptane (V. Santoro and G. Spadaccini, Ric. Sci., 38, XI (1968). (b) We believe that, at present, the problem of predicting the fragmentation pattern of an ion-molecule is still open. The attempt to correlate fragmentation patterns to charge distributions has not yet proved t o be satisfactory. Everybody knows that another most interesting approach has been developed by Rosenstock and coworkers (H. M. Rosenstock, M. B. Wallenstein, A. L. Wahrhaftig, and H. Eyring, Proc. Nat. Acad. Sei. U.S., 38, 667 (1962), and the following papers), the so-called “quasi-equilibrium theory.” Recently, Spiteller and coworkers (G. Spiteller and M. SpitellerFriedmann, Ann. Chem., 690, 1 (1966), and the following papers) have suggested another quite different kind of approach. A serious difficulty to test any theoretical prediction is the scarcity of really reliable experimental information about fragmentation patterns.
Sir: From the previous communication by Santoro and Spadaccini, it appears that there is some misunderstanding concerning our early paper,’ presumably due to overconciseness. More recent papers apparently did not come to the attention of these authors. However, since the point raised by them was only briefly dealt with in these latter papersl2v3a restatement of our present position may therefore be appropriate. It is obvious that a meaningful comparison between the charge density diagram of a molecular ion and the observed mass spectrum can only be made in those cases where secondary dissociations do not play a dominant role. This can be achieved by lowering the energy of the impinging electrons, i.e., lowering the internal energy of the molecular ion. This is the reason why, in the case of branched alkanes, our paper1 includes spectra taken with low-energy electrons. Moreover, for the molecular ions we have studied, further decomposition is probably not very important (in contradistinction to the case of the n-alkanes) because the primary dissociation gives rather stable C3 and C4ions, as noted by Santoro and Spadaccini. It is clear that, for larger branched alkanes, such as those studied by the latter authors, secondary dissociation becomes important, and a direct comparison with the spectra taken a t electron energies around 50 eV has little significance. This is the reason why such compounds were not studied in ref 1. In the case of the n-alkanes, the mass spectrum observed at 50 eV is indeed “the initial fragmentation pattern strongly distorted by further secondary, tertiary, etc., unimolecular decompositions of the initially formed i ~ n s . ” ~ However, J the recent experimental technique developed by Spiteller and Spiteller-Friedmanri,* which consists of recording the mass spectrum at low energies of the electrons together with a low temperature of the ion source, eliminates this complication. The mass spectrum is then the image of the initial fragmentation, and can be explained by a random dissociation of the CC bonds of the molecular ion, in agreement with the calculated charge distribution. lleyersons and Stevenson and Schissler2 independently arrived a t the same conclusion. The agreement is also good in the
(1) J. C. Lorquet, N o Z . Phys., 9 , 101 (1965). (2) J. C. Lorquet in “Advances in Mass Spectrometry,” W. L. Mead, Ed., The Institute of Petroleum, London, 1966. See especially the comment by D. P.Stevenson, p 448 ff. ISTITUTO DI FISICA SUPERIORE VITTORIA SANTORO OF NAPLES UNIVERSITY GIULIOSPADACCINI (3) J. C. Lorquet, MBm. SOC.Roy. Sci. LiBge, XVI, 30 (1968). (4) G. Spiteller and 34, Spiteller-Friedmann, Liebigs Ann. Chsm., NAPLES, ITALY 690, 1 (1965). RECEIVED JULY22, 1968 (6) s. Meyerson, J. Chem. Phys., 42, 2181 (1961). Volume YS, Number 0 February 1.969
