Photochemical substitution reactions of 4-Pyridinecarbonitrile with aliphatic alcohols in neutral and acidified medium

Chemical evidence is given that the title photo-reactions proceed via hydrogen abstraction by the photo-excited base from the alcohol in neutral medium and via electron-transfer process between the photo-excited base and the alcohol in hydrochloric acid acidified medium.     

AB initio calculations of the electronic properties of N4− in KN3

The energies of the ground state and low-lying excited states of the N₄^? defect in KN₃ have been calculated using ab initio techniques. A rectangular equilibrium geometry with dimensions X = 2.76 and Z = 2.47 a.u. and ground state symmetry of Γ₄⁺ was determined by calculating N₄^? as a free radical. For this ground state the unpaired electron is in a π orbital which is consistent with the experimental hyperfine tensor only if one edge of the N₄^? radical is parallel to the c axis in KN₃. These results were used to calculate the X²Γ₄⁺ state of N₄^? in the crystal field of KN₃, yielding an energy of ?217.899 Hartrees. The isotropic hyperfine constant was calculated to be a = 2.1 G and the components of the anisotropic hyperfine tensor as B_xx = ?3.4 G, B_yy = 7.0 G and B_zz = ?3.6 G, in good agreement with experimental and INDO results. Several excited states were calculated for the N₄^? defect in KN₃. When an estimate was made of the correlation energy, the transition energy of the X²Γ₄⁺→A²Γ₃^? transition agreed well with the peak energy of the 780 nm absorption band which has been attributed to N₄^?.     

A two-dimensional model of the distribution of trace gases in the stratosphere and troposphere

Summary We report the results of computations of the distribution of trace gases in the troposphere and stratosphere with a two-dimensional photochemical transport model. The region covered by the model extends from pole to pole and from 0 to 50 km. Atmospheric transport is included by using seasonal averaged values for mean meridional and vertical winds and eddy diffusion. The compounds transported in the model are N₂O, NO _x (NO+NO₂), NO _y (NO_x+HNO₃), N₂O₅, H₂O, CH₄ and H₂O₂. Odd hydrogen (OH+HO₂+H) is assumed in photoequilibrium. A simple scheme is proposed to simulate the condensation of H₂O and scavenging of other gases. Rayleigh scattering is included in the calculation of the photodissociation rates. The resulting distributions are compared whenever possible with experimental data. We attempt to discuss the stratospheric budget of water vapour taking into account photochemical sources (e.g. CH₄ oxidation) and sinks. It is shown that similarly to other models serious disagreement seems to exist between the calculated and observed mixing ratios of NO _x , NO _y and the ratios of the different compound within the family. The amount of experimental data is, however, too sparse to draw any definite conclusion.

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Riassunto Si riportano i rusultati sulla distribuzione di gas minori nella stratosfera e troposfera, ottenuti con un modello bidimensionale, fotochimico con trasporto. Il modello si estende da 0 a 50 km in quota e da polo a polo in latitudine. Del trasporto si tiene conto usando i valori medi stagionali dei venti meridionali e verticali e dei coefficienti di diffusione turbolenta. Il modello tratta il trasporto per i seguenti composti: N₂O, NO _x (NO+NO₂), NO _y (NO _x +HNO₃), N₂O₅, H₂O, CH₄ e H₂O₂. L'idrogeno dispari (H+OH+HO₄) è calcolato all'equilibrio fotochimico. Un semplice meccanismo è proposto per simulare la condensazione del vapor d'acqua e di altri gas. I coefficienti di fotodissociazione sono calcolati tenendo conto dello scattering di Rayleigh. Le distribuzioni ottenute per i vari gas sono confrontate, ove ciò è possible, con i dati sperimentali. Si discute inoltre il bilancio stratosferico del vapor d'acqua tenendo conto delle sorgenti fotochimiche (cioè l'ossidazione del CH₄) e dei pozzi. Si conclude che, analogamente ad altri modelli, esistono delle importanti differenze fra i rappporti di mescolamento calcolati e qualli osservati di NO _x e NO _y . I dati sperimentali sono comunque ancora troppo scarsi per arrivare a conclusioni definitive.

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Резюме Мы приводим результаты вычислений распределения прослеживаемых газов в тропосфере и стратосфере с помощью двумерной фотохимической модели переноса. Область, которую покрывает модель, распостраняется от полюса до полюса и от нуля до 50 км. Атмосферный перенос включает сезонные усредненные значения для средних меридиональных и вертикальных ветров и диффзию вихрей. Переносимые соединения в этой модели являются: N₂O, NO _x (NO+NO₂), NO _y (NO _x +HNO₃), N₂O₅, H₂O, CH₄ и H₂O₂. Предполагается, что нечетный водород (OH+HO₂+H) находится в фоторавновесии. Предлагается простая схема для моделирования конденсации H₂O и удаления другин газов. При вьчислении интенсивностей фотодиссоциации учитывается релеевское рассеяние. Полученные распределения сравниваются с имеющимися экспериментальными данными. Мы пытаемся обсудить стратосферный бюджет водяных паров, учитывая фотохимические источники (т.е. окисление CH₄) и стоки. Показывается, что аналогично другим моделям, по-видимому, существует существенное расхождение между вычисленными и измеренными отношениями перемешивания NO _x , NO _y и отношениями для различных соединений. Однако экспериментальные данные являются слишком разбросанными, чтобы сделать определенные выводы.

     

Regioselective synthesis of trisubstituted 2,3-dihydrofurans from donor-acceptor cyclopropanes or from reaction of the Corey ylide with alpha-sulfenyl-, alpha-sulfinyl-, or alpha-sulfonylenones

[reactions: see text] Regioselective synthesis of 2,4,5- or 3,4,5-trisubstituted 2,3-dihydrofurans has been realized by using donor-acceptor cyclopropanes or by a Corey ylide reaction with alpha-sulfenyl-, alpha-sulfinyl-, or alpha-sulfonylenones. The method allowed a straightforward synthesis of the natural product calyxolane B.     

Complete single-photon counting and timing module in a microchip

A complete module for single-photon counting and timing is demonstrated in a single chip. Features comparable with or better than commercially available macroscopic modules are obtained by integration of an active-quenching and active-reset circuit in complementary metal-oxide semiconductor technology together with a single-photon avalanche diode (SPAD). The integrated SPAD has a 12-microm-diameter sensitive area and operates with an overvoltage above breakdown adjustable up to 20 V. With a 5-V overvoltage the photon detection efficiency peaks above 40% around 500 nm, and the dark-counting rate is lower than 600 counts/s at room temperature. The overall counting dead time is 33 ns.     

Scaling exponents and probability distributions of DNA end-to-end distance

The scaling of the average gyration radius of polymers as a function of their length can be experimentally determined from ensemble measurements, such as light scattering, and agrees with analytical estimates. Ensemble techniques, yet, do not give access to the full probability distributions. Single molecule techniques, instead, can deliver information on both average quantities and distribution functions. Here we exploit the high resolution of atomic force microscopy over long DNA molecules adsorbed on a surface to measure the average end-to-end distance as a function of the DNA length, and its full distribution function. We find that all the scaling exponents are close to the predicted 3D values (upsilon=0.589+/-0.006 and delta=2.58+/-0.77). These results suggest that the adsorption process is akin to a geometric projection from 3D to 2D, known to preserve the scaling properties of fractal objects of dimension df<2.