Delta Hf Research Articles

Blunted heart rate response to vagal withdrawal in persons with tetraplegia

Cardiovascular autonomic mechanisms control heart rate (HR) and determination of heart rate variability (HRV) permits the quantitative assessment of relative shifts in autonomic cardiac control during head-up tilt (HUT). The study herein used HRV techniques to determine the vagal and sympathetic contribution to the change in HR during HUT in persons with tetraplegia (T; n = 7) paraplegia (P; n = 7) and a non-spinal cord injured (non-SCI; n = 8) control group. Heart rate (HR) was continuously monitored and cardiovascular autonomic responses were assessed for 5-minutes at supine and at 45 degrees HUT. Change associated with tilt from supine to 45 degrees HUT was calculated for HR (deltaHR), high & low frequency HRV (HF & LF) and the LF/HF ratio. HR and LF power were lower in the T compared to the P and non-SCI groups at 45 degrees HUT, whereas there were no group differences for HF at 45 degrees HUT. The LF/HF ratio was lower in the T compared to the non-SCI group at 45 degrees HUT. The relationship between delta HR and delta HF response differed between groups (significant group x delta HF interaction) such that the slope of this relationship was reduced in the T (-1.026: 95% CI: -2.623 to 0.571) compared with the non-SCI (-6.985: 95% CI: -11.25 to 2.72) and P (-5.218: 95% CI: -8.197 to -2.239) groups. There was no significant interaction effect for the relationships between deltaHR and deltaLF or deltaLF/HF. In summary, although the magnitude of vagal withdrawal was comparable among the groups, the increase in HR was attenuated in the group with tetraplegia, which may reflect reduced sympathetic cardiac modulation or altered SA node responsiveness to vagal withdrawal.

The heat of formation of chlorine-isocyanate and the relative stability of isoelectronic molecules: An experimental and theoretical study

Accurate thermochemical data of small molecules are invaluable to the progress of every aspect of chemistry, especially in the atmosphere, combustion and industry. In this work, photofragmentation translational spectroscopy and 1st principles electronic structure theory reveal the literature value of the heat of formation of chlorine-isocyanate to be in error by more than 40 kcalmol. We report a revised experimental value for D0(Cl-NCO) = 51+/-3 kcal/mol which leads to a Delta Hf (ClNCO) = 8.5+/-3 kcal/mol. High level ab initio (CCSD(T)) electronic structure calculations extrapolated to the complete basis set limit give D0(Cl-NCO) = 6.3 kcal/mol, in good agreement with experiment. In light of the present results, the destabilization of azides relative to isoelectronic isocyanates has been evaluated empirically for three pairs of related molecules. It is found to be 90-110 kcal/mol, and has been attributed mainly to the weakening of the N-NN bond relative to the N-CO bond. Electronic structure calculations employing decomposition analysis suggest that, compared to homopolar N2, the (+delta)CO(-delta) pi polarity provides better orbital interaction (charge transfer) and electrostatic attraction and results in a closer encounter and larger stabilization between the fragments and that this is the origin of isoelectronic destabilization of azides relative to the isocyanates.

Heats of Formation and Singlet−Triplet Separations of Hydroxymethylene and 1-Hydroxyethylidene

Thermochemical parameters of hydroxymethylene (HC:OH) and 1-hydroxyethylidene (CH3C:OH) were evaluated by using coupled-cluster, CCSD(T), theory, in conjunction with the augmented correlation consistent, aug-cc-pVnZ, basis sets, with n = D, T, Q, and 5, extrapolated to the complete basis set limit. The predicted value at 298 K for Delta Hf(CH2O) is -26.0 +/- 1 kcal/mol, as compared to an experimental value of -25.98 +/- 0.01 kcal/mol, and for Delta Hf(CH:OH) it is 26.1 +/- 1 kcal/mol. The hydroxymethylene-formaldehyde energy gap is 52.1 +/- 0.5 kcal/mol, the singlet-triplet separation of hydroxymethylene is Delta E(ST)(HC:OH) = 25.3 +/- 0.5 kcal/mol, the proton affinity is PA(HC:OH) = 222.5 +/- 0.5 kcal/mol, and the ionization energy is IEa(HC:OH) = 8.91 +/- 0.04 eV. The predicted value at 298 K for Delta Hf(CH3CHO) is -39.1 +/- 1 kcal/mol as compared to an experimental value of -40.80 +/- 0.35 kcal/mol, and for Delta Hf(CH3C:OH) it is 11.2 +/- 1 kcal/mol. The hydroxyethylidene-acetaldehyde energy gap is 50.6 +/- 0.5 kcal/mol, the singlet-triplet separation of 1-hydroxyethylidene is Delta E(ST)(CH3C:OH) = 30.5 +/- 0.5 kcal/mol, the proton affinity is PA(CH3C:OH) = 234.7 +/- 0.5 kcal/mol, and the ionization energy is IEa(CH3C:OH) = 8.18 +/- 0.04 eV. The calculated energy differences between the carbene and aldehyde isomers, and, thus, the heats of formation of the carbenes, differ from the experimental values by 2.5 kcal/mol.

Thermodynamic Properties of Molecular Borane Amines and the [BH4-][NH4+] Salt for Chemical Hydrogen Storage Systems from ab Initio Electronic Structure Theory

The heats of formation for the borane amines BH3NH3, BH2NH2, and HBNH, tetrahedral BH4-, and the BN molecule have been calculated by using ab initio molecular orbital theory. Coupled cluster calculations with single and double excitations and perturbative triples (CCSD(T)) were employed for the total valence electronic energies. Correlation consistent basis sets were used, up through the augmented quadruple-zeta, to extrapolate to the complete basis set limit. Core/valence, scalar relativistic, and spin-orbit corrections were included in an additive fashion to predict the atomization energies. Geometries were calculated at the CCSD(T) level up through at least aug-cc-pVTZ and frequencies were calculated at the CCSD(T)/aug-cc-pVDZ level. The heats of formation (in kcal/mol) at 0 K in the gas phase are Delta Hf(BH3NH3) = -9.1, Delta Hf(BH2NH2) = -15.9, Delta Hf(BHNH) = 13.6, Delta Hf(BN) = 146.4, and Delta Hf(BH4-) = -11.6. The reported experimental value for Delta Hf(BN) is clearly in error. The heat of formation of the salt [BH4-][NH4+](s) has been estimated by using an empirical expression for the lattice energy and the calculated heats of formation of the two component ions. The calculations show that both BH3NH3(g) and [BH4-][NH4+](s) can serve as good hydrogen storage systems which release H2 in a slightly exothermic process. The hydride affinity of BH3 is calculated to be 72.2 kcal/mol, in excellent agreement with the experimental value at 298 K of 74.2 +/- 2.8 kcal/mol.

Regioselectivity and quantitative structure-activity relationships for the conjugation of a series of fluoronitrobenzenes by purified glutathione S-transferase enzymes from rat and man.

Quantitative structure-activity relationships (QSAR's) are described for the rate of conjugation of a series of fluoronitrobenzenes with cytosolic as well as with two major alpha and mu class enzymes of rat and human liver, viz., glutathione S-transferases (GST) 1-1, 3-3, A1-1, and M1a-1a. For all purified enzymes studied, the natural logarithm of the rate of conversion of the fluoronitrobenzenes correlates with both the calculated reactivity of the fluoronitrobenzenes for an electrophilic attack (i.e., E(LUMO)) and the calculated relative heat of formation for formation of the respective Meisenheimer complex intermediate (delta delta HF). In addition, the regioselectivity of the reaction was determined and compared. The results obtained strongly support the conclusion that chemical reactivity of the fluoronitrobenzenes is the main factor determining the outcomes of their conversion by all glutathione S-transferase enzymes. The regioselectivities vary only a few percent from one enzyme to another, whereas QSAR lines for all purified enzymes are in the same region and run parallel. This indicates that in the overall reaction the nucleophilic attack of the thiolate anion on the fluoronitrobenzenes, leading to formation of the Meisenheimer complex, is the rate-limiting step in the overall catalysis. The fact that chemical reactivity of the fluoronitrobenzenes is the main factor in setting the outcomes of the overall conversion by the different glutathione S-transferase enzymes implies that extrapolation from rat to results of other species including man, and also from one individual to another, must be feasible. That this is actually the case is clearly demonstrated by the results of the present study.

Quantitative structure-activity relationships based on computer calculated parameters for the overall rate of glutathione S-transferase catalyzed conjugation of a series of fluoronitrobenzenes.

The present study describes quantitative structure--activity relationships (QSAR's) for the overall rate of conjugation of a series of fluoronitrobenzenes catalyzed by cytosolic glutathione S-transferases based on experimental data and outcomes of computer calculations. The natural logarithm of the rate of conjugation of the series of fluoronitrobenzenes correlates (r = 0.986) with the calculated energy (E) of their lowest unoccupied molecular orbital (LUMO) and also (r = -0.987) with the relative heat of formation (delta delta HF) for formation of the Meisenheimer complex of the fluoronitrobenzenes with a MeS- model nucleophile. In addition, the paper describes QSAR's for the chemical reaction of glutathione with the fluorinated nitrobenzenes both at pH 7.6 and at pH 9.9. These QSAR's are parallel to the one obtained for the enzyme catalyzed conversions. This indicates that in the overall reaction (both chemical and enzyme catalyzed) the interaction between the thiolate anion of glutathione and the fluoronitrobenzene leading to the Meisenheimer reaction intermediate is the rate-limiting step in overall conversion of these substrates. The parallel QSAR's of the chemical and enzymatic reaction also indicate that in the enzymatic reaction chemical reactivity parameters determine the overall outcome of catalysis and, in addition, that the chemical and enzymatic reactions proceed through a similar reaction pathway with comparable reaction intermediates. Additional results of the present study demonstrate that the regioselectivity of the glutathione conjugation cannot be explained on the basis of calculated characteristics of the LUMO of the fluoronitrobenzenes or the delta delta HF for the formation of their Meisenheimer reaction complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Formation and decay of (3PJ)O atoms in the laser flash photolysis of chlorine dioxide (OC10) at 308 nm

The quantum yields of O(3P(J)) and Cl(2P(3/2)) atoms released in the laser flash photolysis of OClO at 308 and 298 K were determined and the kinetics of the subsequent oxygen atom decay was investigated using time-resolved atomic resonance fluorescence measurements. The results are consistent with the formation of sym-ClO3 having a Delta Hf(ClO3) (formed in the reaction O + OClO + Ar) of 55.6 +/-4 kcal/mol.

Raman study of NaClO3 as a function of temperature into the melt and the novel high temperature phase

Raman spectra have been measured for oriented single crystals of NaClO3 II from 77 K to the melting temperature, for the molten salt and the high temperature phase, NaClO3 I. The results for NaClO3 II were in excellent agreement with unit-cell group analysis. Apparent anomalies associated with the presence of weak bands due to naturally abundant 37Cl and 18O substituted ions have been accounted for on the basis of intermolecular coupling models. The unusual high temperature, ambient pressure phase transition for NaClO3 has been characterised by spectroscopic and thermal methods. The novel feature about the high temperature phase is the fact that it may only be reached by cooling the melt and it transforms back to the room temperature phase at about 510 K as the sample is cooled. The longitudinal optical mode at about 1030 cm-1 which was characteristic of the acentric room temperature phase (P213, T4) was conspicuously absent in the Raman spectrum of the high temperature phase. The Raman spectrum of the high temperature phase indicated that the crystal has a centric structure but the similarity of many bands with the spectrum of the room temperature phase suggested that the phase transition involved only small changes in atomic positions. The structure remained ordered but there was evidence of considerable band broadening due to increased thermal motion. The Raman results are consistent with the diffraction studies of Meyer and Gasperin (1973) who reported a monoclinic structure similar to room temperature KClO3, space group P21/a. Measurements of the heats of fusion and transition by differential scanning calorimetry gave values of Delta Hf=21.7 kJ mol-1, Delta Sf=40.6 J mol-1, Delta Ht=4.1 kJ mol-1, Delta St=8.2 J mol-1 K-1. The relatively small values of the heat and entropy of transition compared to those of fusion also indicated that the high temperature phase was not disordered.

Kinetics and Thermodynamics of the Reaction of Deoxyhemerythrin with Nitric Oxide.

Deoxyhemerythrin reacts with NO to form a 1:1 adduct shown spectrophotometrically. The kinetics of the formation have been studied directly by stopped-flow measurements at four different temperatures (0.0-23.6 degrees C). The kinetics of the dissociation have been studied, also by stopped-flow techniques, at five different temperatures (4.0-35.1 degrees C) using three different scavengers [Fe(II)(edta)2-, O2 and sperm whale deoxymyoglobin], which gave similar values. For the formation kf = (4.2 +/- 0.2) x 10(6) M-1 s-1, delta Hf not equal = 44.3 +/- 2.3 kJ mol-1, delta Sf not equal to = 30 +/- 8 J mol-1 K-1 and for the dissociation kd = 0.84 +/- 0.02 s-1, delta Hd not equal to 95.6 +/- 2.1 kJ mol-1 delta Sd not equal to = 74 +/- 7 J mol-1 K-1 (25 degrees C, I = 0.2 M and pH 7-8.1). From the kinetic data the thermodynamic data for the formation of HrNO were calculated: Kf = (5.0 +/- 0.3) x 10(6) M-1, delta H = -51.3 +/- 3.1 kJ mol-1 and delta S = -44 +/- 11 J mol-1 K-1 (25 degrees C). The kinetic data suggest that NO occupies the same iron(II) site in deoxyhemerythrin as oxygen does. The equilibrium constant for the formation of Fe(II)(edta)(NO)2- has been redetermined: K1 = (1.45 +/- 0.07) x 10(7) M-1, delta H = -77.5 +/- 1.5 kJ and mol-1 and delta S = -123.5 J mol-1 K-1 (25 degrees C).

Sympathetic predominance in essential hypertension: a study employing spectral analysis of heart rate variability.

In this study on 91 subjects we tested the hypothesis of an enhanced sympathetic activity in uncomplicated essential hypertension employing spectral analysis of heart rate variability. With this technique the tonic sympathetic and vagal activities and their changes are respectively assessed by the power of approximately 0.1 Hz (low frequency, LF) and approximately 0.25 Hz (respiratory linked, high frequency, HF) components of the spectrum of the beat by beat variability of RR interval. When comparing the 40 subjects with diastolic blood pressure consistently greater than 95 mmHg (hypertensives, Ht), with 35 age-matched controls (diastolic arterial pressure less than 90 mmHg, Nt), we observed that LF was greater and HF smaller in Ht as compared to Nt, thus suggesting an enhanced sympathetic activity and a reduce vagal activity in Ht. Additionally, passive tilt, which in Nt enhances LF [delta = 26 +/- 2 normalized units (nu)] and reduces HF (delta = -22 +/- 2, nu), produced smaller (P less than 0.05) changes in Ht (delta LF = 6.3 +/- 2.7 and delta HF = -7.5 +/- 2.3 nu). Furthermore, the values of LF at rest and the altered effects of tilt on LF and HF were significantly correlated with the degree of the hypertensive state. Chronic beta-adrenergic blockade (atenolol 100 mg once daily for 2 weeks, n = 13) reduced heart rate and blood pressure (from 162/103 to 136/88 mmHg) together with a significant diminution of LF and an increase of HF. Thus, spectral analysis of RR variability appears to be a convenient non-invasive technique to follow the progressive alterations in sympatho-vagal balance present in essential hypertension.

Interaction of benzene with bilayers. Thermal and structural studies.

The thermal and structural properties of saturated phosphatidylcholine liposomes are significantly altered by benzene. Upon the addition of benzene, the liposomes first swell and then disperse into small multilamellar vesicles. At 20 degrees C these vesicles contain striations or ripples in the plane of the bilayer. Major changes in the thermal behavior of DSPC-benzene liposomes occur near mole ratios of 2:1 and 1:1. At a 2:1 mole ratio, the area under the main endothermic peak, delta Hm, essentially disappears; however, the total heat absorbed, delta Hf, remains approximately equal to that of the control. This occurs because for benzene mole fractions 0.12 less than x less than 0.50, benzene increases the apparent molar heat capacity, Cp, of the gel phase to about 1.2 kcal/(mol . deg). We interpret this increase in heat capacity to be due to an increase in the concentration of defects (or disorder) in the gel phase. At mole fractions of benzene between 0.5 and 0.9, the transition temperature decreases by 20-30 degrees C, and broad, multiple transitions are observed. From 0.5 less than or equal to x less than or equal to 0.9, the apparent molar heat capacity of the liquid-crystal phase increases to that of the defected rippled gel phase. The value of delta Hf approaches the heat of fusion for 2 mol of n-octadecane, suggesting that benzene uncouples the liquid-crystalline acyl chains. The lipids affected by benzene or "boundary lipids" have higher heat capacity than nonperturbed lipids. The apparent molar specific heat, Cp, of 1,2-distearoyl-sn-glycero-3-phosphorylcholine (and 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine) multilamellar vesicles is 0.20 +/- 0.05 kcal/(mol. deg) in the L beta', P beta, and L alpha phases. Cp fluctuates about this value in all three phases upon repeated phase transitions in the same sample. However, the value of Cp in the P beta (rippled) phase exhibits much greater fluctuations in Cp than that in the L alpha phase. We attribute these fluctuations to crystal packing defects.

FLUORINE BOMB CALORIMETRY. III. THE HEAT OF FORMATION OF BORON TRIFLUORIDE1-3

The heat of formation of boron trifluoride was measured by direct combination of the elements in a bomb calorimeter. DELTA Hf f at 25 f was found to be -270.10 el temperatures 0.24 kcal. mole -1.