Intermolecular Clamping by Hydrogen Bonds: 2-Pyridone center dot NH3

A combined spectroscopic and ab initio theoretical study of the doubly hydrogen-bonded complex of 2-pyridone (2PY) with NH3 has been performed. The S-1 <- S-0 spectrum extends up to approximate to 1200 cm(-1) above the 0(0)(0) band, close to twice the range observed for 2PY. The S-1 state nonradiative decay for vibrations above approximate to 300 cm(-1) in the NH3 complex is dramatically slowed down relative to bare 2PY. Also, the Delta v=2,4,... overtone bands of the v(1)' and v(2)' out-of-plane vibrations that dominate the low-energy spectral region of 2PY are much weaker or missing for 2PY center dot NH3, which implies that the bridging (2PY)NH center dot center dot center dot NH3 and H2NH center dot center dot center dot O=C H-bonds clamp the 2PY at a planar geometry in the S-1 state. The mass-resolved UV vibronic spectra of jet-cooled 2PY center dot NH3 and its H/D mixed isotopomers are measured using two-color resonant two-photon ionization spectroscopy. The S-0 and S-1 equilibrium structures and normal-mode frequencies are calculated by density functional (B3LYP) and correlated ab initio methods (MP2 and approximate second-order coupled-cluster, CC2). The S-1 <- S-0 vibronic assignments are based on configuration interaction singles (CIS) and CC2 calculations. A doubly H-bonded bridged structure of C-S symmetry is predicted, in agreement with that of Held and Pratt [J. Am. Chem. Soc. 1993, 115, 9718]. While the B3LYP and MP2 calculated rotational constants are in very good agreement with experiment, the calculated H2NH center dot center dot center dot O=C H-bond distance is approximate to 0.7 angstrom shorter than that derived by Held and Pratt. On the other hand, this underlines their observation that ammonia can act as a strong H-bond donor when built into an H-bonded bridge. The CC2 calculations predict the H2NH center dot center dot center dot O distance to increase by 0.2 angstrom upon S-1 <- S-0 electronic excitation, while the (2PY)NH center dot center dot center dot NH3 H-bond remains nearly unchanged. Thus, the expansion of the doubly H-bonded bridge in the excited state is asymmetric and almost wholly due to the weakening of the interaction of ammonia with the keto acceptor group.

Syntheses, structures and magnetic properties of two copper(Ii) diphosphonates: [NH3(CH2)2NH3]2[Cu2(hedp)2]·H2O and [NH3CH(CH3)CH2NH3]2[Cu2(hedp)2] (hedp = 1-hydroxyethylidenediphosphonate)

This paper describes the syntheses and characterization of two new copper(II) diphosphonates: [NH3(CH2)2NH3]2[Cu2(hedp)2]·H2O (1) and [NH3CH(CH3)CH2NH3]2[Cu2(hedp)2] (2) (hedp = 1-hydroxyethylidenediphosphonate). Both compounds exhibit similar one-dimensional linear chain structures. The symmetrical {Cu2(hedp)2} dimers are connected by edge-shared {CuO5} square pyramids and form infinite chains. The Cu(II) ions are alternately bridged by O–P–O groups and O atoms. The Cu–O–Cu angles are 95.8(1) and 96.1(1)° for 1 and 2, respectively. Their magnetic properties show moderately strong antiferromagnetic interactions in both compounds.

N budget and NH3 exchange of a grass/clover crop at two levels of N application

Atmospheric ammonia (NH3) exchange during a single growing season was measured over two grass/clover fields managed by cutting and treated with different rates of mineral nitrogen (N) fertilizer. The aim was to quantify the total NH3 exchange of the two systems in relation to their N budget, the latter was split into N derived from symbiotic fixation, from fertilization, and from the soil. The experimental site was located in an intensively managed agricultural area on the Swiss plateau. Two adjacent fields with mixtures of perennial ryegrass (Lolium perenne L.), cocks foot (Dactylis glomerata L.), white clover (Trifolium repens L.) and red clover (Trifolium pratense L.) were used. These were treated with either 80 or 160 kg N ha−1 applied as NH4NO3 fertilizer in equal portions after each of four cuts. Continuous NH3 flux measurements were carried out by micrometeorological techniques. To determine the contribution of each species to the overall NH3 canopy compensation point, stomatal NH3 compensation points of the individual plant species were determined on the basis of NH4+ + NH3 (NHx) concentrations and pH in the apoplast. Symbiotic N2 fixation was measured by the 15N dilution method.