Showing papers by "Christopher C. Cummins published in 2013"

Thermodynamic and kinetic study of cleavage of the N-O bond of N-oxides by a vanadium(III) complex: enhanced oxygen atom transfer reaction rates for adducts of nitrous oxide and mesityl nitrile oxide.

Abstract: Thermodynamic, kinetic, and computational studies are reported for oxygen atom transfer (OAT) to the complex V(N[t-Bu]Ar)3 (Ar = 3,5-C6H3Me2, 1) from compounds containing N–O bonds with a range of BDEs spanning nearly 100 kcal mol–1: PhNO (108) > SIPr/MesCNO (75) > PyO (63) > IPr/N2O (62) > MesCNO (53) > N2O (40) > dbabhNO (10) (Mes = mesityl; SIPr = 1,3-bis(diisopropyl)phenylimidazolin-2-ylidene; Py = pyridine; IPr = 1,3-bis(diisopropyl)phenylimidazol-2-ylidene; dbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene). Stopped flow kinetic studies of the OAT reactions show a range of kinetic behavior influenced by both the mode and strength of coordination of the O donor and its ease of atom transfer. Four categories of kinetic behavior are observed depending upon the magnitudes of the rate constants involved: (I) dinuclear OAT following an overall third order rate law (N2O); (II) formation of stable oxidant-bound complexes followed by OAT in a separate step (PyO and PhNO); (III) transient formation a...

Relaxation and Dissociation Following Photoexcitation of the (μ-N2)[Mo(N[t-Bu]Ar)3]2 Dinitrogen Cleavage Intermediate

Abstract: Frequency resolved pump–probe spectroscopy was performed on isolated (μ-N2)[Mo(N[t-Bu]Ar)3]2 (Ar = 3,5-C6H3Me2), an intermediate formed in the reaction of Mo(N[t-Bu]Ar)3 to bind and cleave dinitrogen. Evidence is presented for 300 fs internal conversion followed by subpicosecond vibrational cooling on the ground electronic state in competition with bond dissociation. Fast cooling following photoexcitation leads to a relatively low overall dissociation yield of 5%, in quantitative agreement with previous work [Curley, J. J.; Cooke, T. R.; Reece, S. Y.; Mueller, P.; Cummins, C. C. J. Am. Chem. Soc.2008, 130, 9394]. Coupling of vibrational modes to the excitation and internal conversion results in a nonthermal distribution of energy following conversion, and this provides sufficient bias to allow the nitrogen cleavage reaction to compete with breaking of the Mo–NN bond despite a higher energetic barrier on the ground state.

Functionalization reactions characteristic of a robust bicyclic diphosphane framework.

Abstract: The 3,4,8,9-tetramethyl-1,6-diphospha-bicyclo-[4.4.0]deca-3,8-diene (P2(C6H10)2) framework containing a P-P bond has allowed for an unprecedented selectivity toward functionalization of a single phosphorus lone pair with reference to acyclic diphosphane molecules. Functionalization at the second phosphorus atom was found to proceed at a significantly slower rate, thus opening the pathway for obtaining mixed functional groups for a pair of P-P bonded λ(5)-phosphorus atoms. Reactivity with the chalcogen-atom donors MesCNO (Mes = 2,4,6-C6H2Me3) and SSbPh3 has allowed for the selective synthesis of the diphosphane chalcogenides OP2(C6H10)2 (87%), O2P2(C6H10)2 (94%), SP2(C6H10)2 (56%), and S2P2(C6H10)2 (87%). Computational studies indicate that the oxygen-atom transfer reactions involve penta-coordinated phosphorus intermediates that have four-membered {PONC} cycles. The P-E bond dissociation enthalpies in EP2(C6H10)2 were measured via calorimetric studies to be 134.7 ± 2.1 kcal/mol for P-O, and 93 ± 3 kcal/mol for P-S, respectively, in good agreement with the computed values. Additional reactivity with breaking of the P-P bond and formation of diphosphinate O3P2(C6H10)2 was only observed to occur upon heating of dimethylsulfoxide solutions of the precursor. Reactivity of diphosphane P2(C6H10)2 with azides allowed the isolation of monoiminophosphoranes (RN)P2(C6H10)2(R = Mes, CPh3, SiMe3), and treatment with additional MesN3 yielded symmetric and unsymmetric diiminodiphosphoranes (RN)(MesN)P2(C6H10)2 (91% for R = Mes). Metalation reactions with the bulky diiminodiphosphorane ligand (MesN)2P2(C6H10)2 (nppn) allowed for the isolation and characterization of (nppn)Mo(η(3)-C3H5)Cl(CO)2 (91%), (nppn)NiCl2 (76%), and [(nppn)Ni(η(3)-2-C3H4Me)][OTf] showing that these ligands provide an attractive preorganized binding pocket for both late and early transition metals.

Backgrounding and accommodation of presuppositions: an experimental approach

Abstract: Recent research on presupposition has aimed to use techniques of experimental semantics and pragmatics to cast light on the processes that underlie projection and information packaging. Relatively little attention has so far been paid to the relation between the diversity of presuppositions with respect to information packaging and their projection behaviour. In this paper, we argue that information backgrounding and projection can be seen as closely related phenomena, and we present an experimental study investigating the behaviour of a variety of presupposition triggers. We interpret the results as evidence for the psychological reality of at least one of the theoretical distinctions between presupposition types posited in the literature (lexical versus resolution presuppositions), and consider their implications for the competing accounts of presupposition projection.

A pathway to diphosphorus from the dissociation of photoexcited tetraphosphorus

Abstract: We report a computational study of an energetically favorable pathway for the excited-state dissociation of a tetrahedral P4 molecule into two P2 molecules via the simultaneous breaking of four chemical bonds along a highly symmetric (D2d) reaction pathway. Along this pathway, a degeneracy occurs between the first excited state of P4 and the ground state of 2P2 at a lower total energy (ca. 4.7 eV) than the initial state, indicating that the initial photoexcitation provides sufficient energy for the dissociation without significant kinetic barriers. We also found that sequential dissociation of the four P–P bonds exhibits larger activation barriers thus making this a less viable dissociation pathway. Our computational investigation uncovers complicated photochemistry in elemental phosphorus, and suggests a likely mechanism for the environmentally friendly inclusion of phosphorus atoms into organic molecules.

Forward modelling requires intention recognition and non-impoverished predictions.

Abstract: We encourage Pickering & Garrod (P&G) to implement this promising theory in a computational model. The proposed theory crucially relies on having an efficient and reliable mechanism for early intention recognition. Furthermore, the generation of impoverished predictions is incompatible with a number of key phenomena that motivated P&G's theory. Explaining these phenomena requires fully specified perceptual predictions in both comprehension and production.

Priming and conceptual pacts in overhearers’ adoption of referring expressions

Abstract: Priming and Conceptual Pacts in Overhearers’ Adoption of Referring Expressions Abstract Current theories of communication yield predictions about the expression choice of overhearers as well as primary discourse participants. We discuss three such theories and evaluate them with reference to new data on object naming elicited through a confederate priming paradigm. Our results show that participants adopt primed referring expressions if they are highly involved in the task, but mere exposure to the object labels yields very limited priming effects. Also, common ground is a relatively marginal factor in expression choice here. We interpret these results as supportive of the importance of grounding and challenging for interactive alignment-based accounts of expression choice. Introduction Inter-personal communication is customarily taken to involve processes of cooperation and coordination between interlocutors at a number of levels. At the level of the conversational turn, speakers cooperate with hearers by making their contribution appropriate to the current purpose of the talk exchange, as observed by Grice (1975). They also coordinate with hearers by making their intentions understood (Grice 1957). At a discourse level, speaker and hearer work together to achieve conversational goals, which might involve the sharing of information, the making and satisfying of requests, the formation of joint plans, etc. (Clark 1996). A diverse range of theories have been proposed to account for how speakers and listeners successfully engage in this process of communication, with particular reference to dyadic interactions such as dialogues. An influential account of dialogue, the interactive-alignment model (Pickering & Garrod 2004), places low-level processes of priming at the heart of communication. In this account, interlocutors align their representations as a result of dialogue. This alignment commences at a surface level, in that the dialogue participants converge at a lexical and syntactic level, due to the priming effects exerted by the use of particular words and syntactic forms. The resulting alignment then percolates up through the system, eventually reaching the level of situation models. The goal of communication, on this account, is to accomplish the alignment of situation models. However, the fundamental drivers of this are low-level, automatic and unconscious processes, specifically priming processes. Consequently, this account posits little involvement of strategic factors in the success of dyadic communication. A contrary viewpoint is that interlocutors are highly aware of each other’s mental states and that this awareness informs their behaviour. Clark and Schaefer (1989) argue that successful contributions to a discourse requires grounding; that is, speaker and hearer must mutually believe that the speaker’s meaning has been understood. The notion of common ground (CG) – the shared knowledge, beliefs and assumptions of the interlocutors (Clark & Marshall 1981) – thus becomes relevant here. The goal of the interaction involves building and updating CG, and doing this requires consideration of the existing CG state. A simple example is the use of a referring expression: if a speaker predicates a new property of an entity (e.g. “John is away”), this can only be successful as a discourse contribution if the hearer correctly identifies the entity. This requires the speaker to take account of the hearer’s knowledge about how this entity is labelled. Such knowledge may be presumed on the basis of linguistic community membership, but it may also arise from previous referential success, or on the basis of the formation of “conceptual pacts” (Brennan & Clark 1996). Distinctively, conceptual pacts involve the establishment of partner-specific labels for entities, which can then be successfully used in interaction with that specific partner but are not preferred for general use with other interlocutors. An intermediate position between these two viewpoints is occupied by Keysar (2007). He argues that “when people communicate, they do not routinely take into account the mental states of others” (ibid., p.72). Instead, drawing upon evidence from Theory of Mind experiments, he argues for the primacy of egocentric processing, and contends that “one’s own perspective is dominant…the consideration of others’ beliefs is not automatic” (ibid., p.75). Unlike the interactive-alignment model, this approach entails conscious reasoning about the choice of referring expression, but unlike Clark and colleagues, Keysar considers CG to be a relatively peripheral issue, and the role of the hearer and his/her mental state to be a marginal factor in the speaker’s choice of expression. In support of this, Barr and Keysar (2002) provide experimental evidence that people (unconsciously) expect new conversational partners to adhere to conceptual pacts that have previously been established, even though the new partner is not privy to this pact. This in turn suggests that conceptual pacts are not triadic relations between two interlocutors and an entity, in which both agree to refer to this entity in a particular way within their interaction, but rather pairs of relations in which both parties separately agree to refer to this entity by a particular label. Experimental work on dialogue has been conducted from numerous theoretical perspectives, including those outlined above. However, relatively little attention has been paid in this literature to non-dyadic interactions, for instance those in which a third individual is present but not directly engaged in the conversation 1 . In this paper, we aim to extend findings about the choice of referring expression into the domain of non-dyadic interactions. There are several motivations for this move. First, the potential relevance of third parties in conversation has long been acknowledged (see Clark & Carlson 1982) but the implications of this for expression choice have attracted relatively little attention. Secondly, such A partial exception to this is Keysar and Henly (2002), but their primary focus is on using overhearers to evaluate the likely communicative success of utterances, rather than on examining the effect of dialogue on overhearers themselves.

Empirical and theoretical evidence for a model of quantifier production

Abstract: In this chapter we discuss an account of quantifier usage in terms of multiple constraint satisfaction. This model proposes a set of constraints governing the speaker‟s choice of quantified expressions, which are individually motivated by appeal to the experimental literature. By situating these constraints within an Optimality Theory framework, we can draw testable predictions about quantifier usage. Furthermore, assuming that hearers interpret these utterances rationally, it follows that interpretations are also governed by the proposed constraints. We review recent experimental data that validates some predictions of this model. Then we consider its extension to non-numerical quantification, with particular reference to the much-discussed case of scope readings in negatively quantified sentences, and consider how it bears upon the analysis of the critical data. We present new experimental data that test the model‟s applicability to this domain and discuss its implications.

The preference for rounding

Abstract: Various evidence points to a speaker preference for communicating numerical information approximately rather than precisely, e.g. by reporting the time as ‘half past three’ when one’s watch reads 3:27. It has been proposed that the tendency to round reflects a strategy aimed at lowering processing costs for the hearer. Focusing on the domain of clock times, we report on two experiments which demonstrate that round values are in fact recalled and manipulated more quickly and accurately than non- round values, and further that this advantage is due not only to numerical roundness itself, but also to participation on a coarse- grained scale.