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Optical Excitation and Ionization of Adsorbed Molecules.

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nonalternant, isoconjugated and heteroatomic x-electron
systems (polyenes, polyines, aromatic compounds, aryl ethers,
ketones, ketimines) have been synthesized and the relative
energy differences between them have been measured [21:
ground-statechanges by means of vertical ionization potentials
as well as charge-transfer transitions in suitable donoracceptor complexes, and variations in antibonding orbitals
by means of half-wave reduction potentials as well as ESR
coupling constants of radical anions. The total differences
are obtained from the electronic spectra.
The experimental results can be interpreted by different
inductive polarization + ISiR, > + I C R ~ and an additional
electron back-donation Si(d) + C,, which also explain
other characteristic properties of the molecules, such as
stretching and deformation vibrational frequencies,lH-N MR
signals, and dipole moments. Calculations by H M O and
PPP procedures permit correlation and numerical reproduction of the experimental values. Whether the acceptor
function of the R3Si groups simulated in the Si parameters
used are to be ascribed solely to silicon 3d-orbitals and what
degree of importance attaches to the a-states that have so
far been neglected, will be subject of further investigations
on this central problem of inorganic nonmetal chemistry.
Lecture at Braunschweig (Germany) on January 29, 1968 [VB 138 IE]
German version: Angew. Chem. 80, 368 (1968)
[*I Priv.-Doz. Dr. H. Bock
Institut fur Anorganische Chemie der Universitlt
8 Munchen 2, Meiserstr. 1 (Germany)
[l] H . Bock, G . Rudor, E. Baltin, and J. Kroner, Angew. Chem.
77, 469 (1965); Angew. Chem. internat. Edit. 4, 457 (1965).
[2] Cf. preliminary communications: H . Bock e f al., Angew.
Chem. 79, 932, 933, 934, 1106; Angew. Chem. internat. Edit. 6,
941, 941, 943, 1085 (1967); Chem. Commun. 1967, 1299.
Optical Excitation and Ionization of Adsorbed
Molecules
By H. Moesta [*I
The lack of spectroscopic information is felt in our still
unsatisfactory ideas about the nature of the bonding in
adsorption. New methods have now been developed for
UV-spectroscopic investigation of chemisorptive bonding.
Technical requirements are the use of ultrahigh vacuum
processes (UHV) and of an extremely powerful light source
that operates also in the short-wave ultraviolet region
(-800 A) under UHV conditions. Further, methods of
detecting changes in the bonding state are needed that permit
measurement with very small samples (some 1010 molecules).
We have studied:
1. Adsorbed alkali atoms on W and Pt as models for the
investigation of the principal processes.
2. Polyatomic gases such as CO, Nz, H2. NH3, HCN, etc., on
metals such as Fe, Pt, and Ni.
3. NaCl molecules on NaCl cleavage planes as model for
non-metallic systems.
Experiments in project (1) were carried out with a UHV
mass spectrometer and a surface-ionization ion source.
Differences in ionization were observed on irradiation with
visible or near-ultraviolet radiation; this dependence o n
wave length can be correlated with adsorption in shifted or
broadened atomic terms of the adsorbate.
Experiments for project (2) have only just begun. An exploding-wire light source has been developed that delivers
monochromatic light with 1016 quanta per flash behind a
monochromator at 1500 A. Changes in the contact potential
were measured for detecting photoreactions of adsorbed
molecules.
Experiments for project (3) concern the rate of evaporation
of NaCl crystals, which was measured in a UHV microbalance,
Angew. Chem. internat. Edit. J Vol. 7 (196%) / No. 5
(i) with illumination and (ii) without illumination.The evaporation corresponds to desorption of NaCl molecules. The
enthalpy of evaporation decreases on irradiation by light in
the region of the ground lattice absorption.
Findings in the three lines of research were discussed in
relation to catalysis and to modern problems of extraterrestrial chemistry.
[VB 137 IE]
Lecture at Koln (Germany) on January 26, 1968
German version: Angew. Chem. 80, 368 (1968)
[*] Doz. Dr. H. Moesta
Institut fur Physikalische Chemie der Universitat
5 3 Bonn, Wegelerstr. 12 (Germany)
Properties of the AUosteric Forms of Yeast
Glyceraldehyde-3-phosphate-dehydrogenase
By K. KirschnerC*J
At p H 8.5 and 40 “ C nicotinamide-adenine dinucleotide
(NAD) is bound to glyceraldehyde-3-phosphate-dehydrogenase (GAPDH) from bakers’ yeast by a cooperative
process; the saturation curve of the tetrameric enzyme has a
sigmoid shape. Temperature-jump relaxation measurements
have shown that this behavior can be described by Monod’s
allosteric mechanism 111. The isomerization of one allosteric
form of the enzyme into the other is so slow (ti/, = 0.1-5 sec)
that various properties of the less affine T form can be
studied with the aid of the “stopped-flow’’ method. After
rapid mixing of apo-GAPDH with NAD the kinetics of the
formation of the enzyme-coenzyme complex can be followed
by its absorption spectrum. The time course is clearly
biphasic. The first, very rapid phase can be interpreted as
the binding of coenzyme to the preferred form of the preequilibrium (4 N A D + TO + T4). In the second, slow phase
the transient species T4 changes into the stable form R4.
The number of binding sites (n’ = 4) and the dissociation
constant K’ of the T form can both be determined by rapid
titration with NAD; the results agree satisfactorily with those
obtained from relaxation measurements. Further, the difference spectrum of the two formsT4 and R4 was measured by the
same procedure: T4 has a weaker absorption band at 360 nm.
The spectrophotometric titration curve was calculated by
means of the extinction coefficients of T4 and R4 obtained in
this way; agreement with experiment was good.
Similar experiments show that, even when all the substrates
are bound (3-phosphoglyceraldehyde. NAD, and arsenate),
the T form is enzymatically inactive. The kinetic results are
supported by the results of rapid spectrophotometric titration
of the SH groups of the enzyme with Ellman’s reagent [5,5’dithiobis(nitrobenzoic acid), DTNB] in the presence and then
i n the absence of NAD. The two allosteric forms T and R
behave differently: the four identical SH groups of the
enzymatically inactive T form react with DTNB eighty times
faster than those of the R form. To a first approximation the
rate constants do not depend on the presence or absence of
NAD. Quantitative evaluation of the SH titration permits a n
independent determination of the characteristic equilibrium
constants LO and L4 and calculation of the theoretical R
function [21; agreement between experiment and theory is
satisfactory.
[VB 140 IE]
Lecture at Gottingen (Germany) on December 14, 1967
German version: Angew. Chem. 80, 369 (1968)
[*I Dr. K. Kirschner
Max-Planck-Institut fur Physikalische Chemie
34 Gottingen, Bunsenstr. 10 (Germany)
[l] K . Kirschner, M . Eigen, R . Bittman, and B. Voigr, Proc. nat.
Acad. Sci. U.S.A. 56, 1661 (1966).
[2] J. Monod, J. Wyman, and J.-P. Changeux, S. molecular Biol.
12, 88 (1965).
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optical, adsorbed, molecules, ionization, excitation
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