Specific heat of UPdâ Siâ

The specific heat of single-crystal U Pd2 Si2 has been studied using both the step heating and continious heating methods for the temperature range 2 to 250 K. Successive phase transitions at Tl = 136I< and T2 = 108I< are reported, which are consistent with current publications. The transition at 40K, which was previously reported, has not been detected. Recent published elastic neutron scattering data, magnetic susceptibility and resistivity results suggest that U Pd2 Si2 may be a heavy fermion compound, however, the electronic specific heat coefficient I (= 18.97 ;~), obtained from the specific heat Cv measurements, is smaller than that of the conventional heavy fermion system. The Debye temperature of U Pd2Si2 is found to be 116.55K. The possibility is discussed that the maximum in CIT in the low-temperature range 2 to 4K corresponds to Schottky anomaly induced by localized magnetic impurities .

Heats of formation of some hydrates /|nA. Phillipson. -- 260 St. Catharines, Ont. : [s. n.],

A great deal of data on the heats of formation of various hydrates has been compiled i n the J.A.N.A.F. and other tables such as the National Bureau of Standards circulars. Comparison of the heat of f ormation of a hydrate with that of the corresponding anhydrate exposes anomalies i n a surprising number of cases. Some of the results are so discordant that i t is apparent that one or the other value is seriously mistaken. No attempt has been made i n this work to determine which value may be correct, but measurements have been made of the difference between these two values. The procedure adopted has been to dissolve the hydrate and the anhydrate, to achieve the same final concentration of the compound in solution, and so to measure the difference in heats of solution .. Measurements were made at OOC in a modified Bunsen ice calorimeter, well insulated and surrounded by an icewater mixture . The observed differences in heats of solut ion were corrected t o 25°0 by using appropriate heat capacity data. These differences offer a direct measure of the enthalpy involved in binding a mole of water into the crystal structure and so should shed light on the nature of binding involved. The following hydrates were studied : MgS04.nH20 (n = 1,4,7), MnC12.nH20 (n = 1, 2), LiI. nH20 (n = 1,3), MnS04. nH20 (n = 1,4), CaC12. nH20 (n = 2,6) , K2C03.1~H20, LiCl.H20, LiBr.2H20, CdC12.2t H2o, and N2H4eH20.

Kinetic and mechanistic studies for the molybdenum- catalyzed oxidation of organic sulfides and olefins by t- DuO2H

This research was focussed on the effects of light, solvent and substituents in the molybdenum-catalyzed oxidation of phenylmethyl sulfides with t-Bu02H and on the effect of light in the molybdenum-catalyzed epoxidation of l-octene with t-Bu02H. It was shown that the Mo(CO)6-catalyzed oxidation of phenylmethyl sulfide with t-Bu02H~ at 35°C, proceeds 278 times faster underUV light than under laboratory lighting, whereas the Mo02(acac)2-catalyzed oxidation proceeds only 1.7 times faster under UV light than under normal laboratory lighting. The difference between the activities of both catalysts was explained by the formation of the catalytically active species, Mo(VI). The formation of the Mo(VI) species, from Mo(CO)6 was observed from the IR spectrum of Mo(CO)6 in the carbonyl region. The Mo(CO)6-catalyzed epoxidation of l-octene with t-Bu02H showed that the reaction proceeded 4.6 times faster under UV light than in the dark or under normal laboratory lighting; the rates of epoxidations were found to be the same in the dark and under normal laboratory lighting. The kinetics of the epoxidations of l-octene with t-Bu02H, catalyzed by Mo02(acac)2 were found to be complicated; after fast initial rates, the epoxidation rates decreased with time. The effect of phenylmethyl sulfide on the Mo(CO)6-catalyzed epoxidation of l-octene waS studied. It was shown that instead of phenylmethyl sulfide, phenylmethyl sulfone, which formed rapidly at 85°C, lowered the reaction rate. The epoxidation of l-octene was found to be 2.5 times faster in benzene than in ethanol. The substituent effect on the Mo02(acac)2-catalyzed oxidations of p-OH, p-CHgO, P-CH3' p-H, p-Cl, p-Br, p-CHgCO, p-HCO and P-N02 substituted phenylmethyl sulfides were studied. The oxidations followed second order kinetics for each case; first order dependency on catalyst concentration was also observed in the oxidation of p-CHgOPhSMeand PhSMe. It was found that electron-donating groups on the para position of phenylmethyl sulfide increased the rate of reaction, while electronwithdrawing groups caused the reaction rate to decrease. The reaction constants 0 were determined by using 0, 0- and 0* constants. The rate effects were paralleled by the activation energies for oxidation. The decomposition of t-Bu02H in the presence of M.o (CO)6, Mo02 (acac)2 and VO(acac)2 was studied. The rates of decomposition were found to be very small compared to the oxidation rates at high concentration of catalysis. The relative rates of the Mo02(acac)2-catalyzed oxidation of p-N02PhSMe by t-Bu02H in the presence of either p-CH30PhSMe or PhSMe clearly show that PhSMe and p-CHgOPhSMe act as co-catalysts in the oxidation of p-N02PhSMe. Benzene, mesity1ene and cyclohexane were used to determine the effect of solvent in the Mo02 (acac)2 and Mo(CO)6-catalyzed oxidation of phenylmethyl sulfide. The results showed that in the absence of hydroxylic solvent, a second molecule of t-Bu02H was involved in the transition state. The complexation of the solvent with the catalyst could not be explained.The oxidations of diphenyl sulfoxide catalyzed by VO(acac)2, Mo(CO)6 and Mo02(acac)2 showed that VO(acac)2 catalyzed the oxidation faster than Mo(CO)6 and Mo02 (acac)2_ Moreover, the Mo(CO)6-catalyzed oxidation of diphenyl sulfoxide proceeded under UV light at 35°C.

Localization of the mechanism of pH-dependent non- photochemical fluorescence quenching in thylakoid membranes

Higher plants have evolved a well-conserved set of photoprotective mechanisms, collectively designated Non-Photochemical Quenching of chlorophyll fluorescence (qN), to deal with the inhibitory absorption of excess light energy by the photosystems. Their main contribution originates from safe thermal deactivation of excited states promoted by a highly-energized thylakoid membrane, detected via lumen acidification. The precise origins of this energy- or LlpH-dependent quenching (qE), arising from either decreased energy transfer efficiency in PSII antennae (~ Young & Frank, 1996; Gilmore & Yamamoto, 1992; Ruban et aI., 1992), from alternative electron transfer pathways in PSII reaction centres (~ Schreiber & Neubauer, 1990; Thompson &Brudvig, 1988; Klimov et aI., 1977), or from both (Wagner et aI., 1996; Walters & Horton, 1993), are a source of considerable controversy. In this study, the origins of qE were investigated in spinach thylakoids using a combination of fluorescence spectroscopic techniques: Pulse Amplitude Modulated (PAM) fluorimetry, pump-probe fluorimetry for the measurement of PSII absorption crosssections, and picosecond fluorescence decay curves fit to a kinetic model for PSII. Quenching by qE (,..,600/0 of maximal fluorescence, Fm) was light-induced in circulating samples and the resulting pH gradient maintained during a dark delay by the lumenacidifying capabilities of thylakoid membrane H+ ATPases. Results for qE were compared to those for the addition of a known antenna quencher, 5-hydroxy-1,4naphthoquinone (5-0H-NQ), titrated to achieve the same degree of Fm quenching as for qE. Quenching of the minimal fluorescence yield, F0' was clear (8 to 130/0) during formation of qE, indicative of classical antenna quenching (Butler, 1984), although the degree was significantly less than that achieved by addition of 5-0H-NQ. Although qE induction resulted in an overall increase in absorption cross-section, unlike the decrease expected for antenna quenchers like the quinone, a larger increase in crosssection was observed when qE induction was attempted in thylakoids with collapsed pH gradients (uncoupled by nigericin), in the absence of xanthophyll cycle operation (inhibited by DTT), or in the absence of quenching (LlpH not maintained in the dark due to omission of ATP). Fluorescence decay curves exhibited a similar disparity between qE-quenched and 5-0H-NQ-quenched thylakoids, although both sets showed accelerated kinetics in the fastest decay components at both F0 and Fm. In addition, the kinetics of dark-adapted thylakoids were nearly identical to those in qEquenched samples at F0' both accelerated in comparison with thylakoids in which the redox poise of the Oxygen-Evolving Complex was randomized by exposure to low levels of background light (which allowed appropriate comparison with F0 yields from quenched samples). When modelled with the Reversible Radical Pair model for PSII (Schatz et aI., 1988), quinone quenching could be sufficiently described by increasing only the rate constant for decay in the antenna (as in Vasil'ev et aI., 1998), whereas modelling of data from qE-quenched thylakoids required changes in both the antenna rate constant and in rate constants for the reaction centre. The clear differences between qE and 5-0H-NQ quenching demonstrated that qE could not have its origins in the antenna alone, but is rather accompanied by reaction centre quenching. Defined mechanisms of reaction centre quenching are discussed, also in relation to the observed post-quenching depression in Fm associated with photoinhibition.

NMR studies of the exchange reactions of CH3CN.BX3 with excess CH3CN /|nJoseph Fogelman. -- 260 St. Catharines, Ont. : [s. n.],

Exchange reactions between molecular complexes and excess acid or base are well known and have been extensively surveyed in the literature(l). Since the exchange mechanism will, in some way involve the breaking of the labile donor-acceptor bond, it follows that a discussion of the factors relating to bonding in molecular complexes will be relevant. In general, a strong Lewis base and a strong Lewis acid form a stable adduct provided that certain stereochemical requirements are met. A strong Lewis base has the following characteristics (1),(2) (i) high electron density at the donor site. (ii) a non-bonded electron pair which has a low ionization potential (iii) electron donating substituents at the donor atom site. (iv) facile approach of the site of the Lewis base to the acceptor site as dictated by the steric hindrance of the substituents. Examples of typical Lewis bases are ethers, nitriles, ketones, alcohols, amines and phosphines. For a strong Lewis acid, the following properties are important:( i) low electron density at the acceptor site. (ii) electron withdrawing substituents. (iii) substituents which do not interfere with the close approach of the Lewis base. (iv) availability of a vacant orbital capable of accepting the lone electron pair of the donor atom. Examples of Lewis acids are the group III and IV halides such (M=B, AI, Ga, In) and MX4 - (M=Si, Ge, Sn, Pb). The relative bond strengths of molecular complexes have been investigated by:- (i) (ii) (iii) (iv) (v] (vi) dipole moment measurements (3). shifts of the carbonyl peaks in the IIIR. (4) ,(5), (6) .. NMR chemical shift data (4),(7),(8),(9). D.V. and visible spectrophotometric shifts (10),(11). equilibrium constant data (12), (13). heats of dissociation and heats of reactions (l~), (16), (17), (18), (19). Many experiments have bben carried out on boron trihalides in order to determine their relative acid strengths. Using pyridine, nitrobenzene, acetonitrile and trimethylamine as reference Lewis bases, it was found that the acid strength varied in order:RBx3 > BC1 3 >BF 3 • For the acetonitrile-boron trihalide and trimethylamine boron trihalide complexes in nitrobenzene, an-NMR study (7) showed that the shift to lower field was. greatest for the BB~3 adduct ~n~ smallest for the BF 3 which is in agreement with the acid strengths. If electronegativities of the substituents were the only important effect, and since c~ Br ,one would expect the electron density at the boron nucleus to vary as BF3of back-bonding varies inversely as the bo~on halogen distance and one would therefore expect the B-F bond to exhibit greater back-bonding character than the B-Cl or B-Br bonds. Since back-bonding transfers electron density from substituent to the boron atom site, this process would be expected to weaken the Lewis acid strength. This explains the Lewis acid strength increasing in the order BF 3 BC1 3 BBr 3 . When the acetonitrile boron trihalide complex is formed, the boron atom undergoes ~_cbange of hybridization from sp2 to sp3. From a linear relationship between the heat of formation of ethyl acetate adducts and the shift in the carbonyl I.R. stretch, Drago (22) et al have proposed that the angular di~tortion of the X-B-X bonds from sp2 (12 ) to sp3 (10 hybridization is proportional to the amount of charge transferred, i.e. to the nature of the base, and they have rejected the earlier concept of reorganization energy in explaining the formation of the adduct bond (19).

Synthesis and properties of some 4H-1, 3, 4 benzothiadiazines /|nby Darko J. Vukov. -- 260 St. Catharines, Ont. : [s. n.],

The work herein has been divided into five sections. In the first section, a new method of converting N-aroyl- hydrazines to hydrazidic halides is described. The second section deals with the products of reaction of hydrazidic halides with thioacetate ion in acetonitrile at room temperature. A number of new acetylthiohydrazides has been isolated together with corresponding hyclrazidic sulphides. Examination of x-ray data for bis-[~ -(2,6- dibromophenylhydrazono) - benZYl] sulphide revealpd the symmetrical structure as the most probable. In the third section, which consists of the three subsections, the synthesis of the 4H-l,3,4 benzothiadiazine ring system has been extended to 4H-l,3,4 benzothiadiazines with substituents in the 5 and 6-positions. Extension of synthesis also involves 4H-l,3,4 benzothiadiazines with mora than one substituent. Nuclear magnetic resonance spectra of 5 and 6 substituted 4H-l,3,4 benzothiadiazines have been ,. recorded. The section ends with a discussion of the mass spectra of some 4H-l.3,4 benzothiadiazines. In the fourth section, which is divided into two sub- -sections, preparation of 7-nitro substituted 4H-l,3,4 benzothiadiazine from N-thiobenzoyl hydrazine and2,4-dinitro -fluorobenzene is found to be satisfactory. Thiohydrazides react with acetic anhydride, in some cases, to give products identical with acetylthiohydrazides obtained from the hydrazidic halides with thioacetate ion at room temperature. In most of the cases thiohydrazides are found to give anomalous products on reaction with acetic anhydride and mechanisms for their formation are discussed. In the fifth section, which forms three subsections, the 4H-l,3,4 benzothiadiazine ring system with a halogen substituent in the 7-position undergoes electrophilic attack preferentially in 5-posi tion. \fuen the 5-posi tion is occupied by a halogen atom, electrophilic substitution occurs at the 7-position of 4H-l,3,4 benzothiadiazine ring system. Substitution at the 4-nitrogen atom in 4H w l,3,4 benzo- -thiadiazine is extremely slow, probably due to delocalisa- -tion of the nitrogen lone pair in the system. Oxidation of 4H-l,3,4 benzothiadiazines occurs at the sulphur atom under relatively mild conditions. t The Appendix deals with the reaction of N-benzoyl-N - -(2,5-dibromophenyl)hydrazine with p-nitrothiophenol~ The proposed p-nitrothiophenoxy - intermediate may undergo benzothiadiazine formation in a proton exchange system.

The mechanism of formation of the mixed boron trihalide adducts of trimenthylamine

Boron trihalide and mixed boron trihalide adducts of trimethylamine have been prepared, and characterized by proton and fluorine N.M.R. spectroscopy. The acceptor power of the boron trihalides was seen to increase in the order BF3 < BC13 < BBr3 < BI3, corroborating previous evidence. The mixed boron trihalides had intermediate Lewis acidities. Solution reactions between adducts and free boron trihalides rapidly led to the formation of mixed adducts when the free boron trihalide is a stronger Lewis acid than that in the adduct. A slower reaction is observed when the free BX3 is a weaker Lewis aoid than that complexed. The mechanism of halogen exchange leading to the mixed (CH3)3NBX3 adducts was investigated. 10B labelling experiments precluded B-N bond rupture as a possible mechanism in solution; results are discussed in terms of halogen-bridged intermediates. Pre-ionization may be important for some systems. At higher temperatures, during gas phase reactions,B-N coordinate bond rupture may be the initial step of reaction. Two mixed adduots, namely (CH3)3NBClBr2 and (CH3)3NBHOIBr were prepared and characterized by Mass Spectrometry