Commensurate Urea Inclusion Crystals With The Guest (E,E)‐1,4-Diiodo-1,3-Butadiene

Published in Crystal Growth & Design (Cryst. Growth Des., 2013, 13 (9), pp. 3852–3855) earlier this year. The theory work is less impressive than the successful crystal growth, with initial solid-state efforts in Crystal09 only very recently now producing good results (leaving the molecular calculations to Gaussian09 in this paper). The procedure leading to the observed crystal structure of this inclusion complex is a significant step in the direction of testing the theory proposed in Bond Alternation In Infinite Periodic Polyacetylene: Dynamical Treatment Of The Anharmonic Potential published earlier this year in J. Mol. Struct.


Caption: Two views along the ba and ca crystal axes of the (E,E)‐1,4-Diiodo-1,3-Butadiene : Urea Inclusion Complex.

Amanda F. Lashua, Tiffany M. Smith, Hegui Hu, Lihui Wei, Damian G. Allis, Michael B. Sponsler, and Bruce S. Hudson

Abstract: The urea inclusion compound (UIC) with (E,E)-1,4-diiodo-1,3-butadiene (DIBD) as a guest (DIBD:UIC) has been prepared and crystallographically characterized at 90 and 298 K as a rare example of a commensurate, fully ordered UIC. The crystal shows nearly hexagonal channels in the monoclinic space group P21/n. The DIBD guest molecules are arranged end-to-end with the nonbonding iodine atoms in the van der Waals contact. The guest structure is compared with that for DIBD at 90 K and with computations for the periodic UIC and isolated DIBD molecule.

NAMOT Pre-Release 2.2.0-pre4 In OSX 10.8 (Maybe Older Versions)

A recent visit to the College of Nanoscale Science and Engineering (CNSE) at SUNY Albany inspired a few new DNA ideas that I decided would be greatly simplified by having NAMOT available again for design. Having failed at the base install of the NAMOT 2 version and, unfortunately, not having NAMOT available in Fink for a simple installation, the solution became to build the pre-release from scratch. Ignoring the many errors one encounters while walking through an OSX/Xcode/Fink/X11 bootstrap, the final procedure worked well and without major problem. As usual, the error messages at varied steps are provided below because, I assume, those messages are what you’re searching for when you find your way here.

0. Required Installations

You’ll need the following installed for this particular build. I believe XCode is the only thing that you’ll have to pay for (if you don’t already have it. I seem to remember paying $5 through the App Store).

Continue reading “NAMOT Pre-Release 2.2.0-pre4 In OSX 10.8 (Maybe Older Versions)”

The Low-/Room-temperature Forms Of The Lithiated Salt Of 3,6-dihydroxy-2,5-dimethoxy-p-benzoquinone: A Combined Experimental And Dispersion-Corrected Density Functional Study

In press, in CrystEngComm (DOI:10.1039/C2CE26523). This is my first full paper completely internet-powered, in that I’ve not physically met any of the other co-authors (also in the internet-powered context, the recent paper on [18]-annulene was written and submitted without sharing a room with Dr. Bruce Hudson, but we’re in the same building, so it doesn’t quite count). Also, one of the few papers for which I had no image generation duties (a rare treat).

The discussion of the very interesting possibilities of molecular redox materials in lithium-ion batteries aside, this paper presents a very thorough example of the power of computational approaches to greatly improve the understanding of solid-state molecular materials by (specifically) 1: overcoming the hydrogen position identification problems inherent in X-ray diffraction methods, 2: reproducing the changes that come with temperature variations in molecular crystals and explaining the origins of those (possibly subtle) changes by way of dispersion-corrected density functional theory, and 3: demonstrating that the nature of intermolecular interactions (specifically hydrogen bonding) can be rigorously cataloged across varied materials using post-optimization tools (in this case, using Carlo Gatti’s excellent TOPOND program).


Caption: Issue cover.

Gaëtan Bonnard, Anne-Lise Barrès, Olivier Mentré, Damian G. Allis, Carlo Gatti, Philippe Poizot and Christine Frayret*


Following our first experimental and computational study of the room temperature (RT) form of the tetrahydrated 3,6-dihydroxy-2,5-dimethoxy-p-benzoquinone (LiM2DHDMQ⋅4H2O) compound, we have researched the occurrence of hydrogen ordering in a new polymorph at lower temperature. The study of polymorphism for the Li2DHDMQ⋅4H2O phase employs both experimental (single crystal X-ray diffraction) and theoretical approaches. While clues for disorder over one bridging water molecule were observed at RT (beta-form),a fully ordered model within a supercell has been evidenced at 100K (alpha-form) and is discussed in conjunction with the features characterizing the first polymorphic form reported previously. Density functional theory (DFT) calculations augmented with an empirical dispersion correction (DFT-D) were applied for the prediction of the structural and chemical bonding properties of the alpha and beta polymorphs of Li2DHDMQ·4H2O. The relative stability of the two polymorphic systems is evidenced. An insight into the interplay of hydrogen bonding, electrostatic and van der Waals (vdW) interactions in affecting the properties of the two polymorphs is gained. This study also shows how information from DFT-D calculations can be used to augment the information from the experimental crystal diffraction pattern and can so play an active role in crystal structure determination, especially by increasing the reliability and accuracy of H-positioning. These more accurate hydrogen coordinates allowed for a quantification of H-bonding strength through a topological analysis of the electron density (Atoms-in-molecules theory).

UNAFold 3.8, MFold Utilities 4.5/4.6 And Additional Component Installation (Using XCode Tools 3 And Fink 0.29.21) For OSX 10.6.x

NOTE: The version numbers for everything are given specifically because aspects of the installation process may change with different versions and, in the event, I will not necessarily know the answer to subsequent problems if major version changes include major changes to the below (and that should clear up the “qualifications” section).

The UNAFold (UNified Nucleic Acid Fold(ing)) nucleic acid folding and hybridization prediction program set (here using version 3.8) can by itself be built with few (and not important) errors in OSX with Xcode Tools 3. The actual running of produces several errors that do not affect the run but do affect the amount/format of the output. It is my assumption that any OS running a less-than “kitchen sink” installation of Linux/Unix (Ubuntu, gentoo and Damn Small Linux come to mind) will have these errors and will require subsequent installations of programs/libraries that pieces of UNAFold rely on for processing output into, specifically, images and PDF files. OSX has the same issue that is easy to handle using Fink (and less so trying to install otherwise completely unrelated programs to make these “dependencies” (programs and libraries) available to UNAFold). Once Fink is installed, it is a few-step process to build UNAFold, move the Mfold Utilities contents to their proper folders (and there is a small trick here as well), and generate a UNAFold-complete install for all your DNA/RNA needs.

Continue reading “UNAFold 3.8, MFold Utilities 4.5/4.6 And Additional Component Installation (Using XCode Tools 3 And Fink 0.29.21) For OSX 10.6.x”

Terahertz Spectroscopic Investigation Of S-(+)-Ketamine Hydrochloride And Vibrational Assignment By Density Functional Theory, “Function Follows Functional Follows Formalism”

Accepted in the Journal of Physical Chemistry A, with my fingers crossed for pulling off the rare double-header in an upcoming print edition of the journal (having missed it by three intermediate articles with the Cs2B12H12 and HMX papers back in 2006 (you’d keep track, too). A fortuitous overlap of scheduled defense dates between P. Hakey, Ph.D. and M. Hudson, A.B.D.). A brief summary of interesting points from this study is provided below, including what I think is a useful point about how to most easily interpret AND represent solid-state vibrational spectra for publications.

1. AS USUAL, YOU CANNOT USE GAS-PHASE CALCULATIONS TO ASSIGN SOLID-STATE TERAHERTZ SPECTRA. It will take a phenomenal piece of data and one helluvan interpretation to convince me otherwise. As a more subtle point (for those attempting an even worse job of vibrational mode assignment), if the molecule exists in its protonated form in the solid-state, do not use the neutral form for your gas-phase calculation (this is a point that came up as part of an MDMA re-assignment published (and posted here) previously).

2. It is very difficult to find what I would consider to be “complete data sets” for molecules and solids being studied by spectroscopic and computational methods. For many molecular solids, the influences of thermal motion are not important to providing a proper vibrational analysis by solid-state density functional theory methods. Heating a crystal may make spectral lines broader, but phase changes and unusual spectral features do not often result when heating a sample from cryogenic (say, liquid nitrogen) to room temperature. Yes, there are thousands of cases where this is not true, but several fold more cases where it is. We are fortunate to live in a temperature regime where characterization is reasonably straightforward and yet we can modify a system to observe its subtle changes under standard laboratory conditions. The THz spectrum of S-(+)-Ketamine Hydrochloride gets a bit cleaner upon cooling, which makes the assignment easier. As the ultimate goal is to be able to characterize these systems in a person’s pocket instead of their liquid nitrogen thermos, the limited observed change to the spectrum upon cooling is important to note.

3. Crystal06 vs. DMol3 – This paper contains what is hoped to be a level, pragmatic discussion about the strengths and weaknesses of computational tools available to terahertz spectroscopists for use in their efforts to assign spectra. This type of discussion is, as a computational chemist using tools and not developing tools, a touchy subject to present on not because of the finger-pointing of limitations with software, but because the Crystal06 team and Accelrys (through Delley’s initial DMol3 code) clearly are doing things that the vast majority of their users (myself included) could in no way do by themselves. The analysis for the theory-minded terahertz spectroscopist is presented comparing two metrics – speed and functionality (specifically, infra-red intensity prediction). What is observed as the baseline is that both DMol3 and Crystal06 make available density functionals and basis sets that, when used at high levels of theory and rigorous convergence criteria, produce simulated terahertz spectra with vibrational mode energies that are in good (if not very good) agreement with each other. For the terahertz spectroscopist, Crystal06 provides as output (although this is system size- and basis set size-dependent) rigorous infrared intensity predictions for vibrational modes, inseparable from mode energy as “the most important” pieces of information for mode assignments. While DMol3 does not produce infrared intensities (the many previous terahertz papers I’ve worked on employed difference-dipole calculations that are, at best, a guesstimate), DMol3 produces very good mode energy predictions in 1/6th to (I’ve seen it happen) 1/10th the time of a comparable Crystal06 calculation. This is the reason DMol3 has been the go-to program for all of the neutron scattering spectroscopy papers cited on this blog (where intensity is determined by normal mode eigenvectors, which are provided by both (and any self-respecting quantum chemical code) programs).

Now, it should be noted that this difference in functionality has NOTHING to do with formalism. Both codes are excellent for what they are intended to do. To the general assignment-minded spectroscopist (the target audience of the Discussion in the paper), any major problem with Crystal06 likely originates with the time to run calculations (and, quite frankly, the time it takes to run a calculation is the worst possible reason for not running a calculation if you need that data. Don’t blame the theory, blame the deadline). In my past exchanges with George Fitzgerald of Accelrys, the issue of DMol3 infrared intensities came up as a feature request that would greatly improve the (this) user experience and Dr. Fitzgerald is very interested (of course) in making a great code that much better. Neither code will be disappearing from my toolbox anytime soon.

4. The Periodicity Of The Molecular Solid Doesn’t Care What The Space Group Is – One of the more significant problems facing the assignment-minded spectroscopist is the physical description of molecular motion in a vibrational mode. In the simplest motions involving the most weakly interacting molecules, translational and rotational motions are often quite easy to pick out and state as such. When the molecules are very weakly interacting, often the intramolecular vibrational modes are easy to identify as well, as they are largely unchanged from their gas-phase descriptions. In ionic solids or strongly hydrogen-bonded systems, it is often much harder to separate out individual molecular motions from “group modes” involving the in- and out-of-phase motions of multiple molecules. In the unit cells of molecular solids, it can be the case that these group modes appear, by inspection, to be extremely complicated, sometimes too involved to easily describe in the confines of a table in a journal article.

S-(+)-Ketamine Hydrochloride is one such example where a great simplification in vibrational mode description comes from thinking, well, “outside the box.” The image below shows two cells and the surrounding molecules of S-(+)-Ketamine Hydrochloride. As it is difficult to see why the mode descriptions are complex from just an image, assume that I am right in this statement of complexity. Part of this complexity comes from the fact that the two molecules in the unit cell are not strongly interacting, instead packed together by van der Waals and dispersion forces more than anything else. The key to a greatly simplified assignment comes from the realization that the most polar fragments of these molecules are aligned on the edges of the unit cell.

An alternate view of molecular vibrational motion comes from considering not the contents of the defined unit cell but the hydrogen-bonding and ionic bonding arrangement that exists between pairs of molecules between unit cells. The colorized image below shows two distinct chains (red and blue) that, when the predicted vibrational modes are animated, become trivial to characterize as the relative motions of a hydrogen/ionic-bonded chain. Rotational motions appear as spinning motions of the chains, translational motions as either chain sliding motions or chain breathing modes. It appears as a larger macromolecule undergoing very “molecular” vibrations. In optical vibrational spectroscopy, selection rules and the unit cell arrangement do not produce in- and out-of-phase motions of the red and blue chains, as only one “chain” exists in the periodicity of the unit cell. In neutron scattering spectroscopy, these relative motions between red and blue would appear in the phonon region. This same discussion was had, in part, in a previous post on the solid-state terahertz assignment of ephedrine (with a nicer picture).

So, look at the cell contents, then see if there’s more structure than crystal packing would indicate. It greatly simplifies the assignment (which, in turn. greatly simplifies the reader’s digestion of the vibrational motions).

Patrick M. Hakey, Damian G. Allis, Matthew R. Hudson, Wayne Ouellette, and Timothy M. Korter

Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100

Abstract: The terahertz (THz) spectrum of (S)-(+)-ketamine hydrochloride has been investigated from 10 to 100 cm-1 (0.3-3.0 THz) at both liquid-nitrogen (78 K) and room (294 K) temperatures. Complete solid-state density functional theory structural analyses and normal-mode analyses are performed using a single hybrid density functional (B3LYP) and three generalized gradient approximation density functionals (BLYP, PBE, PW91). An assignment of the eight features present in the well-resolved cryogenic spectrum is provided based upon solid-state predictions at a PW91/6-31G(d,p) level of theory. The simulations predict that a total of 13 infrared- active vibrational modes contribute to the THz spectrum with 26.4% of the spectral intensity originating from external lattice vibrations.

The Vibrational Spectrum Of Parabanic Acid By Inelastic Neutron Scattering Spectroscopy And Simulation By Solid-State DFT

Available as an ASAP in The Journal of Physical Chemistry A. As a general rule in computational chemistry, the smaller the molecule, the harder it is to get right. As a brief summary, parabanic acid has several interesting properties of significance to computational chemists as both a model for other systems containing similar sub-structures and as a complicated little molecule in its own right.

1. The solid-state spectrum requires solid-state modeling. This should be of no surprise (see the figure below for the difference in solid-state (top) and isolated-molecule (bottom)). This task was undertaken with both DMol3 and Crystal06, with DMol3 calculations responsible for the majority of the analysis of this system (as has always been the case in the neutron studies reported on this site).

2. The agreement in the hydrogen-bonded N-H…O vibrations is, starting from the crystal structure, in poor agreement with experiment. You’ll note the region between 750 and 900 cm-1 is a little too high (and for clarification, the simulated spectrum is in red below). According to the kitchen sink that Matt threw at the structure, the problem is not the same anharmonicity one would acknowledge by Dr. Walnut’s “catalytic handwaving” approach to spectrum assignment (Dr. Walnut does not engage in this behavior, rather endeavors to find it in others where it should not be).

3. The local geometry of the hydrogen-bonding network in this molecular solid leads to notable changes in parabanic acid structure that, in turn, leads to the different behavior of the N-H…O vibrational motions. There is one potentially inflammatory comment in the Conclusions section that results from this identification. The parabanic acid molecule is, at its sub-structure, a set of three constrained peptide linkages that under go subtle but vibrationally-observable changes to their geometry because of crystal packing and intermolecular hydrogen bond formation. This means that the isolated molecule and solid-state forms are different and that peptide groups are influenced by neighboring interactions.

So, why should one care? Suppose one is parameterizing a biomolecular force field (CHARMM, AMBER, GROMOS, etc.) using bond lengths, bond angles, etc., for the amino acid geometry and vibrational data for some aspect of the force constant analysis. The structural data for these force fields often originates with solid-state studies (diffraction results). This means, to those very concerned with structural accuracy, that a geometry we know to be influenced by solid-state interactions is being used as the basis for molecular dynamics calculations that will NOT be used in their solid-state forms. Coupled with the different spectral properties due to intermolecular interactions, the description being used as the basis for the biomolecular force field likely being used in solution (solvent box approaches) is based on data in a phase where the structure and dynamics are altered from their less conformationally-restricted counterpart (in this case, solid-state).

A subtle point, but that’s where applied theoreticians do some of their best work.

Matthew R. Hudson, Damian G. Allis, and Bruce S. Hudson

Department of Chemistry, 1-014 Center for Science and Technology, Syracuse University, Syracuse, New York 13244-4100

Abstract: The incoherent inelastic neutron scattering spectrum of parabanic acid was measured and simulated using solid-state density functional theory (DFT). This molecule was previously the subject of low-temperature X-ray and neutron diffraction studies. While the simulated spectra from several density functionals account for relative intensities and factor group splitting regardless of functional choice, the hydrogen-bending vibrational energies for the out-of-plane modes are poorly described by all methods. The disagreement between calculated and observed out-of-plane hydrogen bending mode energies is examined along with geometry optimization differences of bond lengths, bond angles, and hydrogen-bonding interactions for different functionals. Neutron diffraction suggests nearly symmetric hydrogen atom positions in the crystalline solid for both heavy-atom and N-H bond distances but different hydrogen-bonding angles. The spectroscopic results suggest a significant factor group splitting for the out-of-plane bending motions associated with the hydrogen atoms (N-H) for both the symmetric and asymmetric bending modes, as is also supported by DFT simulations. The differences between the quality of the crystallographic and spectroscopic simulations by isolated-molecule DFT, cluster-based DFT (that account for only the hydrogen-bonding interactions around a single molecule), and solid-state DFT are considered in detail, with parabanic acid serving as an excellent case study due to its small size and the availability of high-quality structure data. These calculations show that hydrogen bonding results in a change in the bond distances and bond angles of parabanic acid from the free molecule values.

L-Alanine Alaninium Nitrate (LAAN) Shout-Out At (And Better Raman Image Here)

It doesn’t happen often.  Simply marking for interested parties that David Bradley wrote a piece about the recent L-Alanine Alaninium Nitrate article published in Physical Chemistry Chemical Physics (Phys. Chem. Chem. Phys., 2009, 11, 9474 – 9483, DOI: 10.1039/b905070a) with a specific focus on the organic ferroelectric behavior of this system as argued from the results of the inelastic neutron scattering (INS) and temperature-dependent Raman spectroscopic studies.  Also, of course, the entire discussion and analysis revolves around the results of the density functional theory (DFT) studies performed on the solid-state system with DMol3.

I find it mildly amusing that a paper that went through several rather exhaustive crystallography-focused review cycles (fighting with crystallography-specific reviewers about the use of the vibrational spectroscopy to provide the more realistic view of this organic salt in the solid-state) makes headlines (well, you know) only for the vibrational spectroscopy.  I certainly won’t point fingers (only browsers), but I’ve yet to see someone say the same of vibrational spectroscopists.

Continue reading “L-Alanine Alaninium Nitrate (LAAN) Shout-Out At (And Better Raman Image Here)”

Low-Temperature X-ray Structure Determination and Inelastic Neutron Scattering Spectroscopic Investigation of L-Alanine Alaninium Nitrate, a Homologue of a Ferroelectric Material

Accepted in Physical Chemistry Chemical Physics. Quite possibly the hardest-fought article within the peer review process I’ve found myself on the revision-side of, with an interesting debate between we authors and two crystallography reviewers occurring over three total revisions (and, it should be noted, two of the three original reviewers accepted the article without any revision, so this really was a debate between we authors and the crystallography reviewers).

To briefly summarize (as the content of the controversy is part of ongoing work), the paper includes an inelastic neutron scattering (INS) spectrum at 25 K, a new X-ray diffraction study at 90 K, and 77 K and 293 K Raman spectra. The INS spectrum and 77 K Raman spectrum contain a feature at 450 cm-1 that does not occur at higher temperature and that is not predicted by the solid-state density functional theory simulation with the 90 K structure. The proposed argument is that a proton is either shifted from an alanine -NH3+ group to a nitrate oxygen (which the crystallography reviewers generally refused to accept as a reasonable explanation) or the potential energy surface for this proton between the -NH3+ and nitrate oxygen is changed considerably due to contraction of the unit cell at low temperature (which our 90 K crystal structure does not show and so, if it occurs, must occur at lower temperature).

Continue reading “Low-Temperature X-ray Structure Determination and Inelastic Neutron Scattering Spectroscopic Investigation of L-Alanine Alaninium Nitrate, a Homologue of a Ferroelectric Material”

Investigation of Crystalline 2-Pyridone Using Terahertz Spectroscopy and Solid-State Density Functional Theory

Accepted in Chemical Physics Letters. A solid-state density functional theory (DFT) follow-up to the solution-phase 2-pyridone (2PD) study published by Motley and Korter previously. Much of the work-up for this paper was straightforward, run-of-the-mill calculation and correlation (on the theory side, anyway). The most difficult part of the analysis was the identification of the easiest way to present the normal mode analysis of the 2PD crystal cell.

In terahertz (THz) spectroscopy, one observes the lowest-frequency vibrational motions of molecules (if the molecule has low-lying vibrational modes, of course). In the solid-state (such as molecular crystals), one observes both low-lying molecular vibrational motions (if they exist) and the relative motions between molecules in the unit cell. The boilerplate separation of internal (intramolecular) and external (between-molecule) modes is performed (and presented) as follows:

A crystal unit cell containing M molecules with N atoms contains 3N-6M internal modes (those modes associated with intramolecular motions), 6M-3 external modes (those modes associated with relative motions between the M molecules, such as rotations and translations), and three acoustic modes.

Some molecules simply do not absorb in the THz region, so all assignments are for external motions (and one simply identifies molecules sliding along axes or spinning around their centers of mass in their lattice site). Some molecules are very strongly bound to neighboring molecules in their lattice sites, which results in significant changes to the mode energies of low-lying vibrational modes (these are far more complicated systems to perform assignments of and a few of these cases are being prepared for future publications). Some molecules are strongly bound in very localized ways in their crystal cell to neighboring molecules and are very weakly bound to other neighbors in other ways. In 2PD, chains of molecules are strongly bound through hydrogen bonding along the crystal c-axis (see the figure below) and only weakly interacting between chains. In the figure below, the blue and red chains are strongly coupled in-chain (hydrogen-bonding) and only weakly coupled (dispersion and van der Waals forces) between chains.

The assignment of the 2PD solid is simplified by two important facts. First, the two chains (red and blue) are related by symmetry (the unit cell contains two anti-parallel 2PD chains). Second, the chains are very weakly interacting.

What point 1 means is that the chains, if in isolation, would undergo the same vibrational motions at the same energies (as if the chains were simply chiral molecules).

What point 2 means is that these chains are, because they interact very weakly, approaching a limit where there can, in fact, be considered isolated chains so that the unit cell will contain vibrational motions that involve the two chains undergoing the same motion in-phase with respect to reach other (in-phase here meaning that, for instance, both of your lungs are expanding at the same time) and out-of-phase with respect each other (the hypothetical case where the left and right lobes are out-of-sync with one another).

For instance, if both chains are sliding along the crystal c-axis in a vibrational mode, that makes the mode the in-phase acoustic translation in c (because the whole cell is sliding in one direction). If the two chains are sliding in opposite directions with respect to each other, this makes the mode the optical translation in c (the center of mass of the cell stays put while the chains undergo out-of-phase motions).

This simplification for the 2PD assignment (and other solid-state molecular chains) turned out to be the mode assignment based on the treatment of not the in-cell contents of atoms and molecular fragments (if we kept ourselves to only viewing what is happening in the cell, for instance), but instead the relative motions of the chains, which requires ever-so-slightly thinking outside of the box.

Tanieka L. Motley, Damian G. Allis, and Timothy M. Korter*

Department of Chemistry, Syracuse University, Syracuse, NY 13244-4100

Crystalline 2-pyridone has been investigated using terahertz vibrational spectroscopy in the range of 10 to 90 cm-1 (0.3 to 2.7 THz). Solid-state density functional theory (B3LYP, BP, and PW91 with the 6-311G(d,p) basis set) was used to simulate and assign both observed terahertz spectral features and a previously published far-infrared spectrum up to 400 cm-1. The PW91 functional was found to provide the best combination of crystal structure and vibrational frequency reproduction. Observed spectral features below 150 cm-1 are assigned to intermolecular movements of the 2-pyridone chains within the unit cell. The use of independent intramolecular and intermolecular frequency scalars is proposed.

The Low-Temperature X-ray Structure, Raman and Inelastic Neutron Scattering Vibrational Spectroscopic Investigation of the Non-centrosymmetric Amino Acid Salt Glycine Lithium Sulfate

Accepted in the Journal of Molecular Structure.  A nice article by the official author (M.R.H.) that combines multiple experimental methodologies with quantum chemical simulations using density functional theory to characterize a molecular inorganic solid with constituents known to have interesting ferroelectric and nonlinear optical (NLO) properties.  We can design remarkably complicated molecules and perform rigorous quantum chemical analyses to tailor properties, but the simple molecules still hold the greatest interest to the application-focused experimentalists (something about being able to make them…).

If this were a terahertz spectroscopy (THz) paper, it would serve as yet another shining example of how one cannot perform isolated-molecule calculations for the assignment of vibrational modes (as the molecules in this system, glycine and sulfate, are THz-transparent).  Relevant to inelastic neutron scattering (INS) and optical (infrared and Raman) spectroscopic techniques, the interesting result of the computational analysis is the predicted overestimation of the energy of the vibrational mode corresponding to the rotation of the –NH3+ groups (in the figure below, nitrogen is in blue, oxygen is in red) in the solid-state.

The question to ask: Is this overestimation in the mode energy a result of (a) the solid-state calculations (BLYP/DNP with DMol3) over-predicting the binding energy of the –NH3+ protons to their hydrogen-bonding proton acceptors (sulfate oxygens being the majority acceptor), (b) expansion of the molecules from their crystal geometries such that the hydrogen atoms are pushed closer to their hydrogen-bond acceptors (so the interaction strength and mode energy is artificially increased because the “oscillator” is smaller), or (c) the use of the harmonic approximation to estimate the shape of the potential for the –NH3+ rotor-esque anharmonic motion (which, in these rotors and similar systems (specifically methyl groups), has been generally seen to be an important (if not occasionally singular) explanation)?

The answer is likely all three.

Matthew R. Hudson, Damian G. Allis, Wayne Ouellette, Patrick M. Hakey and Bruce S. Hudson

Department of Chemistry, 1-014 Center for Science and Technology, Syracuse University, Syracuse, NY 13244-4100, USA

Abstract: The structure of the amino acid salt glycine lithium sulfate (GLS) is determined by X-ray diffraction at 90 K and reveals no significant deviations from the previously reported room temperature structure.  The vibrational spectrum of GLS is measured at 78 and 298 K by Raman spectroscopy and at 25 K by incoherent inelastic neutron scattering (INS) spectroscopy. There is no evidence of a phase transition in the Raman spectra between 78 and 298 K.  Solid-state density functional theory (DFT) is used to simulate the INS spectrum of GLS and to perform a complete normal mode analysis.  Discrepancy between simulation and experiment, namely the anharmonic torsional motion of the –NH3+ functional group at approximately 370 cm-1, is discussed in detail.

Keywords: glycinesulfatodilithium, glycine lithium sulfate, inorganic amino acid salt, nonlinear optical material, vibrational spectroscopy, inelastic neutron scattering spectroscopy, solid-state density functional theory