1758
NOTES
Nitrogen-15 Nuclear Magnetic Resonance Shifts
ready comparison. Since temperature and solution composition were not always specified in the earlier work, we were interested in possible effects of these variables on the 15Nshifts. For a given Me,16NH4-,+ species at constant solution composition we found no measurable change of 15N shift with temperature over ranges of the order of 50". With regard to solution composition, we found that the 15Nshift was essentially independent of Me,15NH4-,+ concentration a t constant total [Cl-] but did vary significantly with total [Cl-] as shown in Figure 1. Substituting LiCl or NaCl for HC1 as the source of additional [Cl-] had a negligible effect and susceptibility corrections were insignificant. The dependence of the 15N shift upon [Cl-] clearly indicates a significant change in the immediate environment of the MeZ1WH4-,+ species with increasing total [Cl-1. The effect diminishes with increasing methyl substitution in accord with the expectation that methyl groups would be more effective than protons in shielding the nitrogen from effects of liquid phase interactions. A small amount of ion-pair formation could reasonably account for the data and observations but other interpretations are possible. Whatever the origin of the change in shift with [Cl-I,the larger discrepancies in Table I cannot be attributed to this cause. Thus, e.g., shifts of -27 ppm (Ogg and Ray4) or -49 ppm (Witanowski and Januszewski8) for Me315NH+lie far
and Coupling Constants for the Methylamine
Hydrochlorides i n Aqueous Solution1 by M. Alei, Jr.,* A. E. Florin, Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico 87644
and W. 14.Litchman2 Department of Chemistry, University of N e w Mexico, Albuquerque, N e w Mexico 87106 (Received November 93, 1970) Publication costs assisted by the Los A l a m o s Scientific Laboratory
In the course of studies of the lSN shifts in ammonia and the methylamine^,^ we became interested in the W shifts for the protonated forms of these compounds. Although both 14Yand 'W shifts for these species have been reported in the literaturej4-*there are large discrepancies between values reported by different workers. We have therefore measured the 16Nshifts for 15XH4+ and all the methylammonium species including Me4lSN+in an effort to obtain a reliable set of values. In the course of this investigation, we also obtained values for Jls" over the series from 16NH4+to M e P N H + . ~
~~
~
~~
~
-~
Table I: I5N Shiftsa and Coupling Constants for Me,WH4-,+ in Aqueous Solutionb Ref 4 shiftsC
14N
I5NH4+ Me16NH8+ Me2WHz+ Me316NH+
Mea15N
+
Ref 5 shifts
14N
- 25
- 21
- 35 -40 - 27 - 53d
-41
Ref 6 shifts
15N
- 24 - 28
Ref 7 shifts
Ref 8 "N shift@
- 23e
-28.5 -32 - 35 -49 -49.5
16N
-441
-Our EN shifts*
rasults-J ~ ~ N H ~
-26.1 -24.5 -26.6 -33.8 -44.7
73.3 75.4 76.1 76.7
c These were derived from pubb The anion is C1- except as otherwise noted. a All shifts are in ppm relative to 16NHa(liq) a t 30". Anion is Br-. lished, calibrated spectra. Values are probably uncertain to h 2 or 3 ppm. d Anion is OH-. ' Anion is NOp-. ' Uncertainty = &0.5-1 ppm. Values measured a t 30" in aqueous solutions 5 M in amine hydrochloride and 1 M in HCl. Uncertainty = &0.3 ppm. Units are Hertz. Uncertainty = h 0 . 3 Hz.
'
In all cases, the shifts were measured by locking the magnetic field and sweeping the frequency. The stability and reproducibility of the system was such that the 15Yshift for a given sample could be reproduced to within z t l HZ (at -6 MHz) over periods of several days even with repeated removal and replacement of the sample in the probe. The various amine hydrochlorides were prepared by standard techniques and their identification and purity confirmed by proton as well as l5Xnmr spectra. Table I lists the 'jN shifts and coupling constants which we measured at 30" in aqueous solutions 5 M in amine hydrochloride and 1 M in HC1. Shifts previously reported in t h e literature are also included for T h e Journal of Physical Chemistry, Vol. 75, -Vo. 11, 1971
outside the range (roughly - 3 2 to -36 ppm) of values which we measured over a wide range of solution com(1) Work supported by the U. 5. Atomic Energy Commission. (2) AWU Faculty Participant at Los Alamos Scientific Laboratory. (3) M. Alei, Jr., A. E. Florin, and W. M. Litchman, J. F. O'Brien, J . P h y s . Chem., 75, 932 (1971). (4) R. A. Ogg and J. D. Ray, J . Chem. Phys., 2 6 , 1339 (1957). ( 5 ) B. M. Schmidt, L. C. Brown, and D. Williams, J . M o l . Spec-
trosc., 2 , 539 (1958). (6) J. C. Lambert, G. Binsch, and J. D. Roberts, Proc. N u t . Acad. Sei. U . S., 51, 735 (1964). (7) D. Herbison-Evans and R. E. Richards, Mol. Phys., 8, 19 (1964). ( 8 ) M. Witanowski and H. Januszewski, Can. J . Chem., 47, 1321
(1969).
1759
NOTES
3 -40
-45
--
I
0
d I 5
1 10
I 15
conductivity measurements have given quite different values, ie., G(0H-) = 0.15and 0A6 In the conductometric pulse radiolysis work carried out in this laboratory for several years,' a number of systems have been encountered which are suitable for the measurement of G(0H-). In view of the difference between published values for G(0H-), we would like to report some of OUT observations. A suitable system consists of a dilute aqueous solution of a scavenger X1 for the hydrated electron and Xz for the OH radical HzO -4 ea,-, H+, OH-, H, OH, Hz, HzOz (1) e,-
I
TOTAL [Cl-1. MOLARITY
Figure 1. Variation of 16Nshift with total chloride concentration in aqueous solutions of amine hydrochlorides at 30".
position. We conclude that the discrepancies must arise either from unreliable referencing or improper identification of the 1\!te,15NH4-,+ species. With regard to the 15NH coupling constants, our values for 15NH4+and CHa15NH3+are in good agreement with those reported by Binsch, Lambert, Roberts, and R ~ b e r t s . Values ~ for (CH3)2'5NHz+and (CH3)315NH+have not, to our knowledge, been previously reported. It appears that the trend to higher values of JWHwith increasing methyl substitution persists throughout the series from 15NH4+to (CHa)a15NH+. A similar trend was previously reported3 for the free amines from lWH3 to (CH3)I5NHbut the increase in Jls" per methyl group for the amines was about three times as great as for the protonated species.
+ XI
--f
XI-
(2)
+ Xz +neutral product P1 H + + OHHzO H + Xz --+neutral product PZ
(sa>
+ Xi +H + + Xi-
(5b)
OH
---f
H
(3)
(4)
After the pulse, an initial increase in conductivity K , is observed which is caused by (H+ OH-) and (H+ XI-), (The concentrations of X1 and Xz are high enough to assure completion of reactions 2, 3, and 5 during the pulse.) K decreases after the pulse until a final value K f is reached. The decrease, which under our conditions occurs with a half-life of 10-30 p e c , is OH-. Kf is theredue to the neutralization of H+ fore the conductivity caused by H + XI-. (XIshould be long lived and the products P1 and Pz should not react with XI-.) The ratio G(0H-)/G(e,,-) can readily be calculated from K ; and ~ t .
+
+
+
+
(9) G. Binsch, J. B. Lambert, B. W. Roberts, and J. D. Roberts, J. Amer. Chem. SOC.,86, 5564 (1964).
The Radiation Chemical Yield of OH- as Determined by Conductometric Pulse Radiolysis
by J. Rabani,' M. Gratzel, S. A. Chaudhri,2 G. Beck, and A. Henglein* Hahn-Meitner-Institut f a r Kernforschuno Berlin, Sektor Strahlenchemie, Berlin-Wannsee (Received December 8.2, 1970) Publication costs assisted by the Hahn-Meitner-lnstitut
Although the formation of OH- ions due to the reaction of OH eaq- in spurs had been considered several years attempts to measure their yield have only recently been made. A total yield of 2.1 per 100 eV in the spurs can be derived from Schwarz' diffusion model.* For the yield of OH- that escape the spurs, two
+
If H reacts according to (5b), G(e,,-) in eq 6 has to be substituted by G(e,,-) G(H). The A values are molar conductivities. This method has also been used by Barker, et aL6 Their scavengers X1 were ions such as Cd2+ and NOz- that cause a rather high base-line conductivity of the solutions. I n our case, nitrobenzene and Oz were used as electron scavengers. Both
+
(1) Visiting professor from the Physical Chemistry Department, Hebrew University, Jerusalem, Israel. (2) Postdoctoral Fellow from the Atomic Energy Commission, Karachi, Pakistan, with a grant from the Alexander v. HumboldtStiftung, Bad Godesberg. (3) (a) M. S. Matheson, Radiat. Res. Suppl., 4, 1 (1964); (b) J. Rabani, ibid., 4 , 71 (1964). (4) H. A. Schwarz, J. Phys. Chem., 73, 1928 (1969). (5) K. H. Schmidt and S. M. Ander, ibid., 73, 2846 (1969). (6) (a) G. C. Barker, P. Fowles, D. C. Sammon, and B. Stringer, Trans. Faraday Sac., 66, 1498 (1970); (b) G. C. Barker, P. Fowles, and B. Stringer, ibid., 66, 1509 (1970); (c) G. C. Barker and P. Fowles, ibid., 66, 1661 (1970). (7) G. Beck, Int. J. Radiat. Phys. Chem., 1, 361 (1969).
T h e Journal of Physical Chemistry, Vol. 76, No. 11, 1971
