Reaction paths of phosphine dissociation on silicon (001).

Using density functional theory and guided by extensive scanning tunneling microscopy (STM) image data, we formulate a detailed mechanism for the dissociation of phosphine (PH3) molecules on the Si(001) surface at room temperature. We distinguish between a main sequence of dissociation that involves PH2+H, PH+2H, and P+3H as observable intermediates, and a secondary sequence that gives rise to PH+H, P+2H, and isolated phosphorus adatoms. The latter sequence arises because PH2 fragments are surprisingly mobile on Si(001) and can diffuse away from the third hydrogen atom that makes up the PH3 stoichiometry. Our calculated activation energies describe the competition between diffusion and dissociation pathways and hence provide a comprehensive model for the numerous adsorbate species observed in STM experiments.

Phosphine dissociation on the Si(001) surface.

Density functional calculations are performed to identify features observed in STM experiments after phosphine (PH3) dosing of the Si(001) surface. On the basis of a comprehensive survey of possible structures, energetics, and simulated STM images, three prominent STM features are assigned to structures containing surface bound PH2, PH, and P, respectively. Collectively, the assigned features outline for the first time a detailed mechanism of PH3 dissociation and P incorporation on Si(001).

An examination of autonomic nervous function in genetically diabetic mice.

1 This study was designed to determine whether the autonomic innervation of the heart and vas deferens in genetically diabetic mice exhibited dysfunction similar to those seen in chemically diabetic animals and diabetic patients. 2 Diabetic mutant mice (outcrossed from the C57 BL/KS db/db strain) were compared with their non-diabetic litter-mates at age 20 to 22 weeks. Right and left atria and vasa deferentia were removed from freshly killed animals and subjected to nerve stimulation and treatment with noradrenaline (NA) or acetylcholine (ACh) in organ baths. 3 Right atria from diabetic animals were less responsive to noradrenergic nerve stimulation than control preparations but there was no such difference between the noradrenergic responses of left atria from the two groups of mice. Both atria were hypersensitive to exogenous NA. 4 Atria from diabetic mice responded to cholinergic nerve stimulation and exogenous ACh in a fashion similar to those of non-diabetic mice. Likewise in the responses of vasa deferentia to nerve stimulation were similar in the two groups. These findings are indicative of some autonomic nervous dysfunction characteristic, to an extent, of diabetes mellitus.

The regrowth of right atrial noradrenergic nerves after 6-hydroxydopamine in genetically diabetic mice; effects of insulin treatment.

1 This study examined the rate of repletion of right atrial noradrenaline levels after a single dose (100 mg/kg i.p.) of 6-hydroxydopamine (6-OH Da) in diabetic and non-diabetic mice of the C57 BL/KS db/db strain. 2 In mice which received no 6-OH Da there was no significant difference, in endogenous noradrenaline levels, between diabetic and non-diabetic animals. The depletion of noradrenaline 24 h after 6-OHDa was slightly more profound in the diabetic mice than in non-diabetic controls. Thereafter the rate of repletion of noradrenaline was more rapid in the diabetic group. 3 The normal noradrenaline content was reinstated in diabetic mice between 7 and 10 days after 6-OHDa. In the non-diabetic group levels similar to those found in untreated mie were not reinstated until 14 days after 6-OHDa. 4 Ten days after 6-OHDa right atria from diabetic mice were markedly more responsive to stimulation of the intramural noradrenergic nerves than were preparations from non-diabetic mice. 5 A group of diabetic mice was treated with insulin (10 m Units/g daily) for 6 weeks. The right atria from these animals, examined 10 days after 6-OHDa, were similar in their responses to noradrenergic nerve stimulation to the preparations from the non-diabetic mice. 6 All these groups of atria gave similar responses to exogenous noradrenaline. These findings indicate that regrowth of noradrenergic terminals after 6-OHDa was more rapid in diabetic mice than in either insulin-treated diabetic mice or non-diabetic mice.

Acquisition of the voicing contrast language-delayed and normal-speaking children.

Voicing is a phonological contrast which emerges early in the speech of children. However, the acoustic correlates of the voicing contrast for stop consonants are fairly complex. In the initial position, voicing is cued primarily by the relative timing of articulatory versus laryngeal gestures. In the final position, the duration of the preceding vowel is associated with the voicing contrast of stop consonants. The purpose of this study was to examine the pattern of acquisition of the voicing contrast in the speech of ten children diagnosed as language-delayed in comparison with the acquisition of the voicing contrast by normal speaking children. The language-delayed and normal-speaking children were matched according to mean length of utterance (MLU) and placed in one of Brown's five developmental stages. Each participant was first given a short test, using natural speech, to determine his or her ability to identify minimal pairs differing in the voicing of stop consonants. Those who passed the test were recorded under standard recording conditions repeating 12 test words. The test words contrasted voiced and voiceless stop consonants in initial and final positions. Spectrograms of the three best productions of each word were used to examine voice-onset time for stops in initial position and preceding vowel duration for stops in final position. Although the language-delayed and normal-speaking children showed equivalent linguistic sophistication (as measured by MLU), the language-delayed children's control of the acoustic-phonetic details of the voicing contrast was less mature than that of the normal-speaking children.

/s/ plus stop clusters in children's speech.

Young children frequently substitute stops for fricative-stop clusters. In this paper, we report acoustic measurements for children's intended voiced and voiceless stops, and for stops substituting for clusters. Judging from voice-onset time measurements, children employ various strategies for assigning phonological status to the substituted stops.

Mechanism of selective action of 3',4'-dichloropropionanilide.

Studies have been carried out to determine the basis for the unique postemergence selective action of the rice herbicide, 3',4'-dichloropropionanilide (DPA), which controls a wide variety of weed species including barnyardgrasses. Absorption studies have shown that both rice (Oryza sativa, L.) and barnyardgrass (Echinochloa crusgalli, L.) foliage absorb DPA readily and equally and, consequently, morphological differences cannot account for its unique selective action. Through the use of uniformly (14)C ring labeled and carbonyl labeled DPA in short-term studies (1-3 days), it has been found that rice and barnyardgrass leaves hydrolyze DPA to 3,4-dichloroaniline (DCA). It is further shown that this is not a direct 1-step hydrolysis, but rather an oxidative metabolism of DPA to 3',4'-dichlorolactanilide (DLA) followed by hydrolysis to DCA and lactic acid. In rice, DLA is a transient intermediate and not isolatable under normal conditions, whereas DLA accumulates in barnyardgrass and is readily isolated. The oxidative metabolism of DPA is rapid in rice but slow in barnyardgrass. Thus, DPA is detoxified rapidly in rice, but accumulates in barnyardgrass to lethal proportions and this difference is proposed as the primary basis for the selective action of DPA.Additional evidence for this mechanism comes from studies on the interaction of certain insecticides with DPA. It is known that the tolerance of rice and other crops to DPA can be completely eliminated when an insecticide such as carbaryl is present with the herbicide in the crop. Under these conditions, it has been found that DLA accumulates in the rice as it does in DPA treated barnyardgrass.

Metabolism of 3',4'-dichloropropionanilide: 3,4-dichloroaniline-lignin complex in rice plants.

Rice plants (Oryza sativa L. var. Bluebonnet 50) metabolize the herbicide 3',4'-dichloropropionanilide to 3,4-dichloroaniline, which in turn conjugates with carbohydrates. Soluble aniline-carbohydrate complexes account for only a small fraction of the hydrolyzed 3',4'-dichloropropionanilide. The major portion of the 3,4-dichloroaniline moiety is found complexed with polymeric cell constituents, mainly lignin. The aniline is lignin-bound as 3,4-dichloroaniline and not as 3',4'-dichloropropionanilide.