The Woessner approach is applied to the 13C relaxation data for tetraphenyltin (1) and tetra(p-tolyl)tin (2) in CDCl3 solution over the temperature range 5-42 degrees C to obtain correlation times for rotational motions and hence the activation barriers. Quantum mechanical computations were carried out to obtain the rotational energy barriers for comparison. For 2 the relaxation data indicate (1) slower ring rotation than in 1, (2) highly hindered internal rotation of the methyl group. IR and chemical shift data support the hypothesis of hyperconjugation of the methyl correlated with interaction between the pi-electrons and the 5d orbitals of tin in the (p-tolyl)Sn moiety to account for the hindrances to the rotations of the ring and the methyl. The activation barrier for the tolyl group rotation is found to be much higher than that for the phenyl rotation. However, the Woessner approach yields an anomalously high barrier for the methyl rotation. An explanation based on correlated rotations of the tolyl ring and the methyl is offered.
NMR measurements ((119)Sn chemical shift, line width and (13)C relaxation) were made on triphenyltin chloride in two solutions, 2.5 and 0.75 mol% in CDCl(3), at several temperatures. The (13)C spin-lattice relaxation time and NOE data for the phenyl carbons provide the corresponding correlation times for the overall molecular reorientational motion and the internal rotation of the phenyl groups. The results are indicative of a weak intermolecular association of the triphenyltin chloride molecules in solution and are discussed with reference to a model for intermolecular phenyl ring π-π stacking interactions.
This paper describes part of a more extensive national study on Malaysian teachers' classroom assessment practices. The main study used a priori coding framework containing four-level performance ratings (i.e., Distinguish, Proficient, Basic, and Unsatisfactory). The findings revealed that a large majority of teachers' practices were at an unsatisfactory level. Among the 30 science teachers, only two teachers were categorised at the Basic level; the rest were at the unsatisfactory level. The present study used a qualitative approach to compare two teachers at basic and unsatisfactory levels and investigated if teachers' social and emotional competencies (SEC) affect the implementation of three Assessment for Learning (AfL) strategies. Three video recordings for each teacher were the main source of data. The findings revealed that the teacher with SEC increased student engagement. The research findings have implications on educational assessment policy and, particularly, teacher education programmes to incorporate elements of SEC to improve teachers' AfL skills.
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