h/*D tetramethylguanidine 25.0 ± 0.6 4.75 ± 0.34 45.8 ± 1.1 14.2 ± 0.2 45 ±2b quinuclidine 63.8 ± 1.5 21.8 ±0.9 17.2 ±0.4 7.20 ± 0.12 15.6 ± 0.6C triethylamine 2.71 ± 0.04 2.08 ± 0.13 24.3 ± 0.5 9.14 ± 0.13 11.0 ± 0.7C tri-n-butylamine 0.675 ± 0.008 0.480 ± 0.032 31.7 ± 0.5 11.0 ± 0.1 14 ± 1°d iethyl-n-butylamidine 12.6 ± 0.2 9.93 ± 0.81 27.1 ± 1.4 9.84 ± 0.36 11.7 ± 1.0d diethyl-zz-nonylamidine 34.9 ± 0.1 19.6 ± 1.2 11.4 ±0.6 5.39 ±0.19 8.0 ± 0.3d pentamethylguanidine 499 ±5 518 ± 13 44.1 ± 1.1 13.8 ± 0.2 13.7 ± 0.4e 0 Estimated by using = &H/2(A:H/fcD)1,442 and published1•10 values of and b Reference lb. c Reference lc. d Reference 10a. e Reference 1 Ob.It was recently pointed out, however, that these large isotope effects could be experimental artifacts if isotopic exchange, and consequent loss of deuterium from the deuterated substrate, occurred under the ionization-reaction conditions.3 Detailed kinetic analysis showed that values of kD would then be erroneously low, and treatment of some data under this assumption reduced very large isotope effects to values of the order of kK/kD = 10-15.4 This criticism was countered by the argument that the rate of hydrogen exchange in this substrate was not known and that internal return5 could easily make exchange much slower than ionization;6 in that eventuality, deuterium would not be lost during
A 50 kV square wave pulse generator and associated control circuitry for use in spectrophotometric dissociation field effect experiments has been constructed. The design differs from previous pulse generators in that cumbersome delay cables are not used. A 0.005 μF plastic capacitor is charged to a voltage adjustable from 30 to 52 kV. When the desired voltage is reached, a triggered spark gap is fired, connecting the capacitor to the chemical cell. At a preset delay time, continuously adjustable from 0 to 29 μsec, a second spark gap is fired, shorting the cell and causing the voltage remaining on the storage capacitor to be discharged through a 100 Ω resistor. This apparatus produces an exceptionally clean pulse with a rise- and fall-time of less than 22 nsec and does not produce the troublesome reflections associated with devices using delay cables. This high voltage pulse perturbs the equilibrium between reacting species in the chemical cell, and the return to equilibrium is observed spectrophotometrically. Chemical relaxation times faster than 30 nsec have been reliably measured with this device.
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