A systematic density functional theory level investigation of differently substituted pyridinium methylides was carried out to determine the role of C(ylidic) lone-pair-associated hyperconjugative and negative hyperconjugative interactions in deciding conformational preferences. Deviation from the coplanar orientation of the carbanionic center with the pyridine ring and its substituent dependence has been found to correlate well with the relative opportunities for conjugative and negative hyperconjugative interactions of a ylidic moiety with different substituent groups present at the ylidic carbon. The contribution of individual n-->pi* conjugative, n-->sigma* negative hyperconjugative, and sigma-->pi* hyperconjugative interactions in a particular conformation of pyridinium dichlorophosphinomethylides was assessed from donor-acceptor stabilization energies, as obtained from natural bond orbital (NBO) analysis. The relative extent of conjugative and negative hyperconjugative interactions with the substituents present at the ylidic carbon plays an important role in permitting the delocalization of ylidic charge into the pyridine ring, thereby controlling the relative orientation of the latter with the carbanionic plane.

A site-selectivity model, based on the Fukui function as a local reactivity descriptor, has been applied to 1,2-disubstituted pyridinium ions incorporating two competing sites of similar reactivity, i.e., 1-methylene and 2-methylene, which may undergo deprotonation depending on the nature of substituent present on these moieties. Applicability of the local HSAB rule, in context with the Li-Evans' generalized HSAB principle suggesting the hard-hard interactions to be controlled by minimum Fukui function, has been illustrated. Global and local reactivity descriptors have been computed by carrying out DFT calculations at B3LYP/6-31++G** level using Mulliken and NPA methods for charge analysis. A comparison with the calculated deprotonation energies involving two sites indicates that the observed site selectivity in differently substituted pyridinium ions is better explained by the Li-Evans rule of minimum Fukui function for hard-hard interactions.

Information on the World Wide Web is increasing tremendously. To get the relevant information from very large data sets is essential. In traditional retrieval systems, the query is given to large corpus to retrieve the relevant documents. The traditional models for information retrieval are just one subclass of retrieval techniques that have been studied in many years. Although many techniques share common characteristics in the information retrieval hierarchy, they all share a core set of similarities that justify their own class and these algorithms are design for isolated datasets. But in most of cases, relationships among different datasets are always existed. A new probabilistic Hidden Markov model is proposed and based on this model new information retrieval (IR) technique is presented. Hidden Markov models (HMMs) are widely used in science, engineering and many other areas. In a HMM, there are two types of states like hidden states and observable states. HMM is powerful modeling of context as well as the current observations. Hidden Markov model is finite state machine which offer a good balance between simplicity and expressiveness of context. IR is performed by determining the sequence of states that was most likely to have generated the entire document, and retrieving the information that were associated with certain designated target states. Determining this sequence is efficiently performed by dynamic programming with the Viterbi algorithm.