Ted M. Clark

Enhancing sensitivity of quadrupolar nuclei in solid-state NMR with multiple rotor assisted population transfers

Rotor-assisted population transfer (RAPT) was developed as a method for enhancing MAS NMR sensitivity of quadrupolar nuclei by transferring polarization associated with satellite transitions to the central m=12-->-12 transition. After a single RAPT transfer, there still remains polarization in the satellite transitions that can be transferred to the central transition. This polarization is available without having to wait for the spin system to return to thermal equilibrium. We describe a new RAPT scheme that uses the remaining polarization of the satellites to obtain a further enhancement of the central transition by performing RAPT-enhanced experiments multiple times before waiting for re-equilibration of the spin system. For 27Al (I=5/2) in albite we obtain a multiple RAPT enhancement of 3.02, a 48% increase over single RAPT. For 93Nb (I=9/2) in NaNbO(3) we obtain a multiple RAPT enhancement of 5.76, an 89% increase over single RAPT. We also describe a data processing procedure for obtaining the maximum possible signal-to-noise ratio.

Dependence of bridging oxygen 17O quadrupolar coupling parameters on Si–O distance and Si–O–Si angle

Ab initio quantum chemistry calculations and comparisons with experimental 17 Os o lid-state nuclear magnetic resonance (NMR) investigations were used to determine the dependence of the 17 O quadrupolar coupling constant and asymmetry parameter on the first-coordination-sphere structure around bridging oxygen. The quadrupolar asymmetry parameter was found to be dependent on the Si–O–Si angle, in agreement with previous studies, and independent of the Si–O distance. In contrast, the quadrupolar coupling constant was found to have as tr ong dependence on Si–O distance as well as Si–O–Si angle. Analytical expressions describing these dependences were proposed and used to develop an approach for relating measured 17 O quadrupolar coupling constant and asymmetry parameter values for bridging oxygen to their Si–O–Si angle and average Si–O distance. Examples of this approach were given using 17 ON MR results from the crystalline silica polymorphs, coesite, α-quartz, cristobalite, and ferrierite. (Some figures in this article are in colour only in the electronic version)

Selective suppression and excitation of solid-state NMR resonances based on quadrupole coupling constants.

The dependence of the (Rotor Assisted Population Transfer) RAPT enhancement on offset frequency for nuclei experiencing different quadrupolar couplings has been exploited to design two new spectral editing schemes, pi/2-RAPT and RAPT-pi-RAPT, for the selective excitation or suppression, respectively, of nuclei with large quadrupolar couplings. Both approaches are demonstrated on the 87 Rb spectrum of Rb(2)SO(4), which contains two resonances with C(q) values of 2.6 and 5.3 MHz. The conditions for optimal selectivity are discussed. Combining pi/2-RAPT with the RIACT MQ-MAS experiment it is also demonstrated how a pure absorption mode triple quantum MQ-MAS spectrum devoid of narrow resonances can be obtained.

Factors influencing the 17O quadrupole coupling constant in bridging oxygen environments

Ab initio calculations were performed on the series of clusters [(OH)3M-O-M(OH)3](-2) where M = B, Al, and Ga, (OH)3M-O-M(OH)3 where M = C, Si, and Ge, (OH)2OM-O-MO(OH)2 where M = P and As, and (OH)O2M-O-MO2(OH) where M = S and Se. These clusters were constructed to model the effect of changing the coordinating cation and coordinating cation-oxygen distances on the bridging oxygen 17O quadrupole coupling constant. For all clusters studied, we observed a general trend that the magnitude of Cq increases linearly with increasing cation-oxygen bond distance and cation group number, suggesting that group number and cation-bridging oxygen distance can serve as a better predictor of the bridging oxygen quadrupole coupling constant than electronegativity differences.

Relationships between bridging oxygen 17O quadrupolar coupling parameters and structure in germanates

We have performed ab initio calculations on the model cluster (OH)3Ge–O–Ge(OH)3 in order to refine the relationships between 17O quadrupolar coupling parameters and the local structure around the bridging oxygen. From these calculations the trend in the bridging oxygen 17O quadrupolar coupling constant, Cq, and asymmetry parameter, ηq, with changing Ge–O–Ge angle was found to be similar to previously established trends in silicate clusters with the overall Cq values being systematically increased in magnitude by approximately 3 MHz. Such a shift can be attributed to differences in cation–oxygen distances as well as coordinating cation electronegativity. In addition, we also investigated the effect of changing intratetrahedron bond angles over a range that has been suggested to exist in germanate systems. While such variations in intratetrahedron bond angles lead to a small variation in Cq, ηq remains relatively unaffected. In such cases, ηq can still be a reliable probe of Ge–O–Ge angle while Cq could serve as a probe of tetrahedron distortions.

Bond length-bond angle correlation in densified silica—Results from 17O NMR spectroscopy

Pressure induced correlated evolution of the distributions of the Si–O distance and Si–O–Si inter-tetrahedral bond angle in vitreous silica quenched from pressures of up to ∼14 GPa at ambient temperature is measured in unprecedented detail using two-dimensional dynamic-angle-spinning 17O nuclear magnetic resonance spectroscopy. The results demonstrate that, in contrast to the conventional wisdom, vitreous silica undergoes irreversible structural changes even at pressures as low as ∼8 GPa. These structural changes at the short range involve a progressive reduction in the mean Si–O–Si angle and a broadening of the corresponding distribution, with increasing pressure. This bond angle reduction is accompanied by a concomitant monotonic increase in the mean Si–O distance. The mean values of the Si–O–Si angle and Si–O distance at various pressures closely follow the minimum in the corresponding potential energy surface calculated for the H6Si2O7 dimer molecule.

Fostering Creativity in Undergraduate Chemistry Courses with In-class Research Projects

Chemical educators have viewed creativity in different ways, ranging from spontaneous “eureka–moments,” to informed hunches, to a skill that may be practiced and improved upon. Intertwined with these descriptions is consideration of what chemists do, how they seek and solve puzzles, and how creativity plays a role. When it comes to educating students in chemistry, traditional laboratory instruction has presented limited opportunities for promoting creativity and decision making. This has begun to change, however. A promising pedagogical approach for fostering creativity in chemistry is the inclusion of in-class research experiences in introductory courses. The Research Experiences to Enhance Learning (REEL) program is an exemplar of a program that has introduced more than 10,000 students to in-class research in Ohio. REEL courses diverge from traditional instruction in many ways, and in this chapter, aspects of the program that foster creativity are discussed. Student perspectives on REEL laboratories are quite positive, with their opportunity to creatively propose and explore research their own questions an important consideration.

Transdisciplinary and Future Directions from Our Conversation Among the Cultures in Science and Art

Although there have been excellent advances in the field of the neurobiology of creativity over the past few decades, further progress is needed. The model proposed by Dr. Flaherty is the most advanced synthesis of the available data thus far, but is only a starting point toward our complete understanding of this aspect of human cognitive neuroscience. The model takes into account the activity of the frontal lobe and the temporal lobe, down the X-axis, along a continuum of normal and abnormal function. Creativity seems to be maximized with normal frontal lobe function, and mild-to-moderate dysfunction of the temporal lobes (i.e., before entering into the realm of mania and psychosis). Along the Y-axis, the level of dopaminergic activity is plotted—another very important neurobiological component in the process of creativity. For creative drive to be maximized, dopaminergic activity must be in the moderate to high range.

Reflections of Our Conversation Among the Cultures in Science and Art

It was very clear from the conversation among all of the dedicated and talented authors of these book chapters in this book, that Creativity was extremely important in all of their fields, even though each of the disciplines was very different, ranging from Art to Music Therapy, to basic fields such as Theoretical Physics and Chemistry, and more clinically oriented fields such as Psychology and Neurology. In each field, it was clear that basic creativity was critical for that field to advance in terms of new research, potential paradigm shifts in large spheres of the field (e.g., Einstein and quantum mechanics in Physics), and to allow for improvements within society as a whole. Even in the purely artistic fields, such as Art and Music, huge leaps in creativity have been necessary for major paradigm shifts to occur. In the Art world, the work of the Impressionists is a perfect example of this kind of creative shift and change that can totally alter the world landscape of a given field. Similarly, for the world of Music, huge creative shifts have occurred over time. For example, when the Age of Rock and Roll was instigated in the 1960s, when young musicians in England discovered the blues music of the American South. Creativity is paramount in these kinds of “leaps ahead” in any given field. However, it is also important for many day-to-day activities that people engage in, that are not recognized as being overly creative, such as teachers figuring out how to “get through” to an unmotivated student, parents helping their children to enjoy reading, and the many ways to apply the tools of modern social media.