Archive for August, 2008

The Solid-State Terahertz Spectrum of MDMA (Ecstasy) – A Unique Test for Molecular Modeling Assignments

Wednesday, August 13th, 2008

In press, in Chemical Physics Letters (CPL). Yes, the blog has taken a bit of a turn, from high explosives to illicit drugs. I expect my google rating to rise sharply with this post. The protonated form of the 3,4-methylene-dioxymethamphetamine (Ecstasy, but I’ll keep the post legit, so it is herein referred to as MDMA) molecule (herein referred to as MDMA:H⁺) and MDMA:H⁺ in its crystal cell with a chloride ion (Cl^-, the crystal form herein referred to as MDMA:HCl) is shown below in yet another fantastic NanoEngineer-1 rendering (if I do say so myself).

This CPL article is, to some extent, a response to those in the terahertz community who continue to attempt spectral assignments of crystalline and poly-crystalline samples using isolated-molecule quantum chemical calculations. The MDMA:HCl sample and MDMA molecule, as a pair, are a very interesting case study of theory and experiment for reasons detailed below. The spectra, shown below from a previous version of the paper (but the same spectra), show quite a bit of detail that will make sense shortly.

Panel A shows the isolated-molecule calculation for the neutral MDMA molecule at a B3LYP/6-31G(d) level of theory (in red). You will note that this simulated spectrum is in very good agreement with experiment (in black), reproducing all of the major features and showing a number of smaller features that account for shoulders. This agreement was the basis for the assignment of the MDMA:HCl spectrum reported in: G Wang, J Shen, Y Jia. “Vibrational spectra of ketamine hydrochloride and 3,4-methylenedioxymethamphetamine in terahertz range.” Journal of Applied Physics 102 (2007) 013106/1-06/4.

The new theoretical analysis reported in the CPL article was instigated by this assignment in this previous publication. Relevant to the measured sample and the previously reported assignment, two points arise that require address.

1. The previous calculation, as reported, was of the neutral MDMA molecule and is reasonably close to the MDMA spectrum shown above (in red. This calculation was redone for the CPL article for comparative purposes). As the experimental THz sample was of solid-state MDMA:HCl, the appropriate form of the molecule to run is not the neutral MDMA molecule, but the protonated form, MDMA:H⁺. The protonated form has a different vibrational spectrum (shown in green in Panel A) than the isolated molecule form. At the very least, the isolated-molecule to consider for the MDMA:HCl sample must be the protonated form. Interestingly, the re-calculation at B3LYP/6-31G(d) reported for the CPL article predicts a fifth vibrational mode at 48.0 cm^-1 that was not reported in the previous study. We do not know if the previous group missed that peak in the write-up, decided that (since it is a low-intensity mode) it was not worth reporting, or if their starting molecular geometry was somehow different so that the other four modes were predicted to be in the same region and this mode was somehow turned off.

2. The solid-state spectrum shown in Panel B at a BP/DNP level of theory does not agree as well as the isolated-molecule MDMA B3LYP/6-31G(d) calculation. That being said, THAT IS NOT THE POINT. The goal of a simulated spectrum IS NOT to obtain the closest spectral agreement with experiment. The goal IS to explain the solid-state spectrum with the best theoretical model possible that, hopefully, is as close to the experimental result as possible. In this case, the solid-state BP/DNP spectrum contains a finite number of vibrational modes that do group according to features in the THz spectrum, making the assignment reasonably straightforward. Interestingly, the two most intense modes in the solid-state BP/DNP calculations involve the motions of the Cl^-…H⁺-N chains, which CANNOT be accounted for in an isolated-molecule calculation of either the neutral MDMA molecule or the protonated MDMA:H⁺.

In summary, as taken from the CPL paper:

With all of these considerations taken into account in this re-examination of the MDMA.HCl THz spectrum, it is found that this system serves as a fortuitous example of one whose THz spectrum is predicted quite precisely by two very different approaches, but is only described accurately by one that considers the crystal environment and the actual state of the molecule in its solid-state form.

Damian G. Allis^1,2, Patrick M. Hakey¹, and Timothy M. Korter¹

1. Department of Chemistry, Syracuse University, Syracuse NY 13244-4100 USA
2. Nanorex, Inc., P.O. Box 7188, Bloomfield Hills, MI 48302-7188 USA

Abstract: The terahertz (THz, far-infrared) spectrum of 3,4-methylene-dioxymethamphetamine hydrochloride (Ecstasy) is simulated using solid-state density functional theory. While a previously reported isolated-molecule calculation is noteworthy for the precision of its solid-state THz reproduction, the solid-state calculation predicts that the isolated-molecule modes account for only half of the spectral features in the THz region, with the remaining structure arising from lattice vibrations that cannot be predicted without solid-state molecular modeling. The molecular origins of the internal mode contributions to the solid-state THz spectrum, as well as the proper consideration of the protonation state of the molecule, are also considered.

NanoEngineer-1, dmol3, gaussian98/03, povray, publications, qc/mm/md progs, quantum chemistry, terahertz (THz) spectroscopy | Comments (2)

Syracuse Astronomical Society President’s Message for July, 2008

Thursday, August 7th, 2008

A repost of the original at the Syracuse Astronomical Society website.

Greetings fellow astrophiles!

Final plans for our Summer Seminar are in the works, with that update to follow early in August. Meantime, with quite a few interesting astronomical stories appearing in notable media these past few weeks, it seems like as good a presentation order as any to start local and head way, way out.

“We soon realized the public is very good at finding weird things.”

Astronomy is one of the few scientific fields where so-called “amateurs” (and today’s good amateur would have been a world authority just 100 years ago) still make numerous significant contributions every year. While some undertake the task of observation with considerable equipment purchases and long nights of tracking objects and recording locations, some need only a web browser and a decent monitor to discover new phenomena.

The Galaxy Zoo (galaxyzoo.org) is an online project established in mid-2007 that combines the mountains of available images of galaxies from the Sloan Digital Sky Survey with a web interface that lets users assist in the cataloguing of galaxy types (elliptical, spiral, mergers, other?). While the first round of results are in the process of being written up for publication, work continues at the Galaxy Zoo site, with the next focus being the identification of merging galaxies. As for the process of identification, it is nice to know (or, at least, have reported on their website) that the human brain is better at classifying galaxies than computer algorithms.

Click to go to galaxyzoo.org.

And how does one stumble upon this site while looking for message content? The answer is Hanny’s Voorwerp. The Astronomy Picture of the Day for June 25, 2008 contains an unusual voorwerp (that’s Dutch for “object”) below the galaxy IC 2497, both about 700 million light years away. While a current hypothesis holds that the voorwerp is a galaxy in close proximity to IC 2497 acting as a reflection nebula, the consensus is that this is a unique object that exemplifies how much odd stuff we have documented that no one’s yet had the time or resources to examine in any detail. The real kicker of this object? Hanny’s Voorwerp was discovered by one of the Galaxy Zoo members in the Netherlands (hence the “voorwerp” title).

So, with Syracuse fall and winter approaching (and, given our luck at viewing sessions so far this year, one might anticipate a very small number of good viewing nights), consider doing some good science indoors instead.

Hanny’s Voorwerp. From APOD, June 25 2008. Click for a larger view.

Another Good Reason To Stay Inside…

While all of these images are on the internet, it is always nice when someone takes the time to organize content into simple web scripts for viewing. It is even better when a major magazine hosts such a series of images, as it provides those not necessarily thinking about astronomical phenomena as part of their morning read an opportunity to think about yet another way they might be late to work the next day. This TIME magazine slideshow begins with photos from Tunguska on June 30, 1908, perhaps the most famous Earth-bound encounter with a celestial object in the 20th century (and an X-Files favorite topic). The additional images of meteors and impact craters seem more otherworldly than they actually are, with the close-crop views of Gosses Bluff and Arizona’s Meteor Crater more reminiscent of images from the Moon, where the lack of constant geological shifting freezes the lunar views, leaving other impacts to cause major changes to the geography.

Tunguska, June 30, 1908. Click HERE to go to the slideshow.

The hunt for Terran impact craters has even expanded into a full-blown hobby thanks to Google Earth, with users scouring satellite images of the entire planet for signs of past meteors impacts.

From “Wanna Take A Ride?” To A Possible “Take A Hike!”

A news item first pointed out by our own Mike Brady. The Arecibo Radio Observatory in Puerto Rico, established in 1963, made famous to many Americans as a backdrop for the beginning of the movie Contact and an episode of the X-Files, and made famous internationally as the original data collector for the SETI@home screensaver project, is facing a severe budget cut by the National Science Foundation. A 60% cut, from 10.5 to 4.0 million by 2011, would mean the closure of this facility as an academic hub of radio astronomy research provided that the extra $6 million cannot be obtained from other sources, which in this day and age could mean checks from individuals instead of corporate sponsorship (because finding the right paint for putting logos might be problematic).

The Arecibo Radio Observatory. Click HERE for more info.

I keep track of this story as part of my membership to the Planetary Society, which provides information and updates as part of their public-accessible website (they are also a major voice in the fight to keep Arecibo running for SETI@Home, Near-Earth Asteroid (NEA) identification, and basic research).

They’d Prefer You Called Them “UFOs”

In yet another article that ties into this little X-Files theme (although I promise that the movie opening this weekend had nothing to do with the selection of stories), WIRED magazine features a story about an art exhibit by Trevor Paglen, astrophotographer extraordinaire (and that includes using astrophotography equipment for more terrestrial art exhibits). Using spy satellite data collected by Ted Molczan, himself featured in a WIRED magazine article in 2006, Paglen set out to take photos of the otherwise non-existent – 189 secret spy satellites. According to the story, “Paglen is trying to draw a metaphorical connection between modern government secrecy and the doctrine of the Catholic Church in Galileo‘s time,” where the lack of acknowledgment of their existence by authorities does nothing to temper the fact that small satellites are orbiting around a larger body (be that the Galilean moons around Jupiter or these spy satellites around the Earth).

This photo does not exist. By Trevor Paglen, www.paglen.com.

… and if you’re not much into government conspiracy and national security issues, they still make for some gorgeous satellite photos.

Deep Impact Looks Back

Taking one giant leap away from the planet, NASA‘s Deep Impact spacecraft has provided the still images for a most amazing movie – the transit of the Moon across the Earth. Deep Impact first made headlines as the spacecraft responsible for cracking open comet Tempel 1 on July 4th, 2005. This newest imagery set of the Earth and Moon has been combined by NASA researchers into a 15 minute-per-frame movie of our Moon transiting the Earth (specifically, passing over Africa with South Americans just beginning to wake up on May 29th of this year).

Deep Impact and the transit of the Moon. From NASA. Click HERE for more info and movies.

While the individual images would be enough with even a mild imagination, the combined series makes for 30 of the best seconds online.

Yeah, Yeah, Yeah, More Water on Mars

It’s becoming mundane! Well, almost. The Nature article that makes up the basis for this story (also from WIRED magazine) reports that Mars was once a far wetter place, with salty seas existing across the entire Martian surface early in its formation (only the first 700 million years, though). While the best Mars has offered us in hard evidence lately is the existence of water ice by the Mars Phoenix Lander, all of the many geological/mineral studies performed over the past few years have provided a very convincing picture of what ancient Mars must have been like before Mother Nature made the “big move” to Earth.

The Nili Fossae region of Mars. From Prof. John Mustard, Brown University. Click HERE for more info.

Jupitero The Magnificent

The great Dissappearing-Reapprearing Little Red Spot Act by Jupiter‘s Great Red Spot at the beginning of July made for a great round of blogging and corrections as the Little Red Spot apparently made it through the Great Red Spot, greatly humbled but still present after its close encounter. A number of real-time blog postings between May 15th and July 8th reported that the Little Red Spot was completely devoured by the Great Red Spot. Only after the final image in the series was collected was it determined that some small deformed remnant of the Little Red Spot made it out the other side of the most famous feature of our largest planetary neighbor.

The Hubble view of the Red Spot event. From NASA and hubblesite.org. Click HERE for more info.

This event was made all the more impressive to amateur astronomers because the entire process occurred while Jupiter was in opposition, putting its best face to Terran observers. Google searches reveal a multitude of images, with the NASA Hubble Space Telescope providing the ultimate views of this most impressive magic show (and as for the size of the performers, note that the Great Red Spot at its narrowest is the same size of the entire planet Earth).

Scientists Scouring The Galaxy For A Stellar Chicken

Now taking a trip across the pond to report on a story about the contents of our entire Solar System, it has been determined after analysis of Voyager 1 and 2 data that our Solar System is egg-shaped. The determination of shape was made by comparing data from Voyager 1 and 2 as they passed through the heliopause, the edge of the magnetic bubble generated by our Sun that gets pushed upon by particles in the interstellar medium. The Voyager 1 and 2 spacecraft, launched in different directions, passed through the heliopause boundary at distances of 8.7 and 7.8 billion miles from the Sun (respectively), a 10% difference in its shape that could only be accounted for by the heliopause being deformed from the perfect spherical shape one would expect in a stationary model of our Solar System and not the 250 km/sec speed of our Solar System’s motion in the Milky Way.

Artist rendering of the Solar system. From telegraph.co.uk. Click on the image for more info.

Come Together… Right Now… p=mv

Finally, in other events beyond our legislative control, our Milky Way galaxy (and a number of our nearby neighbors) is speeding towards… um… something. Something big. Something massive. something 250 millions light years away. Something pulling us toward it at 14 million miles per hour. This massive voorwerp, dubbed “The Great Attractor,” is taking the small suburb we call our Local Group of galaxies and the local metropolis known as the Virgo Cluster for a rapid ride towards an unknown destination, something that is believed to be at least 10 times more massive than all of the visible matter in the massive Virgo cluster. This invisible mass fires the debates about dark matter and dark energy as we continue to develop theories and make observations that enable us to better describe our universe and, hopefully, our place in it.

You are here. Our galactic neighborhood by infra-red imaging. From Thomas Jarrett, IPAC. Click HERE for more info.

This is one of those truly impressive examples of just how much we still have to learn about the universe around us. An impressive read and wikipedia entry I’d recommend anyone gravitate toward.

And finally…

We return to our own local group of medium sized cities with a photo of Lake Ontario in Oswego, NY by our own David Tibbitts, featuring our closest star and more water than you could shake a NASA lander at.

Sunset in Oswego by David Tibbitts. Click the image for a larger view.

Space is the place,
Damian Allis, Ph.D.
sas@somewhereville.com

Links Used Above (Subject To Web Changes)

www.sciencenews.org/view/generic/id/33702/title/Citizen_Astronomy
www.galaxyzoo.org/
www.sdss.org
en.wikipedia.org/wiki/Galaxy
en.wikipedia.org/wiki/Hanny%27s_Voorwerp
apod.nasa.gov/apod
apod.nasa.gov/apod/ap080625.html
www.astr.ua.edu/keel/research/voorwerp.html
apod.nasa.gov/apod/reflection_nebulae.html
www.galaxyzooforum.org/index.php?topic=3802.0
maps.google.com/maps?…geocode_result&resnum=1&ct=title
www.time.com
www.time.com/time/photogallery/0,29307,1818757,00.html
en.wikipedia.org/wiki/Tunguska_event
en.wikipedia.org/wiki/Earth
en.wikipedia.org/wiki/X-files
en.wikipedia.org/wiki/Meteor
en.wikipedia.org/wiki/Impact_crater
en.wikipedia.org/wiki/Gosses_Bluff
en.wikipedia.org/wiki/Arizona
en.wikipedia.org/wiki/Meteor_Crater
en.wikipedia.org/wiki/Moon
earth.google.com
www.naic.edu
en.wikipedia.org/wiki/Puerto_Rico
en.wikipedia.org/wiki/Contact_%28film%29
setiathome.berkeley.edu
www.nsf.gov
en.wikipedia.org/wiki/Arecibo_Observatory
www.planetary.org
www.planetary.org/programs/…/space_advocacy/20080703.html
en.wikipedia.org/wiki/Near_earth_asteroid
www.wired.com
www.wired.com/culture/art/news/2008/06/secret_satellites
www.paglen.com/index.htm
www.wired.com/wired/archive/14.02/spy.html
en.wikipedia.org/wiki/Spy_satellite
en.wikipedia.org/wiki/Catholic_Church
en.wikipedia.org/wiki/Galileo
en.wikipedia.org/wiki/Galilean_moons
en.wikipedia.org/wiki/Jupiter
www.nasa.gov
solarsystem.nasa.gov/deepimpact/index.cfm
www.nasa.gov/topics/solarsystem/features/epoxi_transit.html
en.wikipedia.org/wiki/Deep_Impact_%28space_mission%29
en.wikipedia.org/wiki/9P/Tempel
en.wikipedia.org/wiki/Africa
en.wikipedia.org/wiki/South_America
www.nature.com/index.html
en.wikipedia.org/wiki/Mars
blog.wired.com/wiredscience/2008/07/water-water-eve.html
en.wikipedia.org/wiki/Phoenix_Lander
en.wikipedia.org/wiki/Mother_Nature
en.wikipedia.org/wiki/Nili_Fossae
research.brown.edu/myresearch/John_Mustard
www.brown.edu
blogs.discovermagazine.com/…chews-up-and-spits-out-a-storm/
www.skyandtelescope.com/observing/home/24453169.html?imw=Y
en.wikipedia.org/wiki/Great_Red_Spot
hubblesite.org/newscenter/archive/releases/2008/27/
hubblesite.org
en.wikipedia.org/wiki/Opposition_(astronomy_and_astrology)
www.google.com
www.telegraph.co.uk/earth/…/scisolar103.xml
en.wikipedia.org/wiki/Solar_System
en.wikipedia.org/wiki/Voyager_1
en.wikipedia.org/wiki/Voyager_2
en.wikipedia.org/wiki/Heliopause#Heliopause
en.wikipedia.org/wiki/Sun
en.wikipedia.org/wiki/Milky_Way
telegraph.co.uk
en.wikipedia.org/wiki/Come_together
en.wikipedia.org/wiki/Momentum
www.dailygalaxy.com/my_weblog/2008/07/mystery-of-the.html
en.wikipedia.org/wiki/The_Great_Attractor
en.wikipedia.org/wiki/Local_Group
en.wikipedia.org/wiki/Virgo_cluster
en.wikipedia.org/wiki/Dark_matter
en.wikipedia.org/wiki/Dark_energy
en.wikipedia.org/wiki/Lake_Ontario
www.oswegony.org

astronomy | Comments (0)

Vicinal Deuterium Perturbations on Hydrogen NMR Chemical Shifts in Cyclohexanes

Thursday, August 7th, 2008

In press, in the Journal of the American Chemical Society. To answer a student question (on the first day of class, no less) about the geometry/sp³ hybridization of carbon in my 2nd semester organic chemistry class, Prof. Baldwin began by writing the time-independent Shroedinger Equation on the chalkboard.

Needless to say, the wavefunction that was my future collapsed instantaneously.

To summarize considerable computational effort (and ignoring many, many equations you can find in the paper), the shifts in hydrogen (H) peak positions in the ¹H NMR spectrum of cyclohexane that result from deuteration (D) can be explained by considering the contribution of the vibrational stretching mode differences of the C-H and C-D bonds. Interestingly, the trend determined from the computational studies is fairly insensitive to the use of electron correlation methods (B3LYP, MPW91, MP2 were all employed with 6-31++G(2d,p), 6-311++G(2d,2p), and Aug-cc-pVTZ basis sets), with Restricted Hartree-Fock (RHF) results very similar and much faster to calculate. The RHF results served as the basis for the analysis in the paper.

The geometry optimization of a molecule will provide you static bond lengths (C-H, C-D, C-C for cyclohexane). In the case of just the optimizations, H and D look identical to the electronic wavefunction (the basis of the Born-Oppenheimer Approximation). If the bond lengths stretched like harmonic oscillators (the approximation we almost always use when performing normal mode analyses in quantum chemical codes), the calculated bond lengths would be the correct (physical) average bond lengths. Of course, no chemical bond is a perfect harmonic oscillator, a fact that explains transition probabilities for overtone and combination bands that are forbidden within the harmonic approximation. For atom pairs that stretch with a significant anharmonic character (such as C-H/D), the actual description of the bond length is a very important factor to consider when trying to extract important trends based on the actual bond length behavior, and is as described in the figure below.

This figure shows the bottom of the potential wells for idealized harmonic and Morse oscillators. The figure, like the paper, only considers the contribution from the vibrational zero-point level (v₀). The H_eq/D_eq position at the bottom of the potential wells is the calculated bond length after geometry optimization. If the stretching potential is harmonic (red), the average position of the H/D atom is always at this position regardless of the system energy (the H atom is sampling as much of the short-side of the potential as it is the long-side, which, you guessed it, averages to the middle).

Now consider the anharmonic oscillators (our Morse functions). The H atom, being lighter, has a higher vibrational zero-point energy (VZPE). Its average position (or mean displacement from the C atom) due to VZPE (H_eq + v₀) is slightly longer. The deuterium, with twice the mass, has a lower VZPE. As a result of lying lower along the Morse potential, its average VZPE-corrected position (D_eq + v₀) is closer to the calculated static position than for H. The difference in the contributed length to the average positions is shown in green. This position-based description is really just for show and neglects all of the quantum theory, but it does make the point. You can consider the green to be the effective additional distance within the Morse potential that the two atoms sample compared to the harmonic potential as part of their stretching mode, with the average position differing due to this larger spatial sampling in each oscillation.

So, “real C-D” is shorter than “real C-H”. Accordingly, the ¹H NMR peak position of C-H in the presence of C-H bonds (which are further away) is different than in the presence of C-D bonds (which are closer).

This is also the first paper I’ve been on that did not directly involve my making an image (hence the involved image above).

Daniel J. O’Leary¹, Damian G. Allis^2,3, Bruce S. Hudson², Shelly James², Katherine B. Morgera², and John E. Baldwin²

1. Department of Chemistry, Pomona College, Claremont, California 9171-6338
2. Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100
3. Nanorex, Inc., P.O. Box 7188, Bloomfield Hills, Michigan 48302-7188

Abstract: The substitution of a deuterium for a hydrogen is known to perturb the NMR chemical shift of a neighboring hydrogen atom. The magnitude of such a perturbation may depend on the specifics of bonding and stereochemical relationships within a molecule. For deuterium-labeled cyclohexanes held in a chair conformation at -80^oC or lower, all four possible perturbations of H by D as H-C-C-H is changed to D-C-C-H have been determined experimentally, and the variations seen, ranging from 6.9 to 10.4 ppb, have been calculated from theory and computational methods. The predominant physical origins of the NMR chemical shift perturbations in deuterium-labeled cyclohexanes have been identified and quantified. The trends defined by the Δδ perturbation values obtained through spectroscopic experiments and by theory agree satisfactorily. They do not match the variations typically observed in vicinal JH-H coupling constants as a function of dihedral angles.

gaussian98/03, publications, qc/mm/md progs, quantum chemistry | Comments (0)

Theoretical Analysis of the Solid-State Terahertz Spectrum of the High Explosive RDX

Tuesday, August 5th, 2008

In press, in the journal Chemical Physics Letters. The RDX (cyclotrimethylenetrinitramine) solid-state simulation is the third from the original series of terahertz spectra that served as my ICPRFP research focus. This system is of interest (well, to me anyway) for three reasons.

1. This is the largest DMol³ calculation performed to date for a THz simulation (Z = 8). Between optimization, normal mode analysis, and vibrational mode displacements for the difference dipole intensity calculations, 1.5 solid months on a quad-core box.

2. Zeitler and Taday provided a 7 K THz spectrum of the RDX (in black) to complement the original room temperature spectrum (in blue), providing a far more resolved and feature-rich data set for theoretical comparisons.

3. As obvious from the high-resolution data, the RDX solid-state spectrum contains considerable vibrational structure. In fact, this THz spectrum contains more resolved peaks than there are vibrational modes in the isolated-molecule calculation in this region. While it has been demonstrated in several previous publications that solid-state THz spectra cannot be assigned using isolated-molecule calculations, all of the molecules in the previous studies contained enough vibrational structure in the THz region to potentially let the uninformed researcher attempt to get away with an isolated-molecule assignment. This is not the case for RDX, which contains only 6 isolated-molecule vibrational modes below 125 cm^-1.

Damian G. Allis^1,2, J. Axel Zeitler³, Philip F. Taday⁴, and Timothy M. Korter¹

1. Syracuse University, Department of Chemistry, 1-014 CST, 111 College Place, Syracuse, NY 13244-4100 USA
2. Nanorex, Inc., P.O. Box 7188, Bloomfield Hills, MI 48302-7188 USA
3. Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK
4. TeraView Limited, Platinum Building, St. Johns Innovation Park, Cambridge CB4 0WS, UK

Abstract: The solid-state terahertz (THz) spectrum (2 – 120 cm^-1) of α-form cyclotrimethylenetrinitramine (RDX) has been simulated using solid-state density functional calculations at a BP/DNP level of theory. BP/DNP features are in good agreement with both 298 K and a new 7 K polycrystalline RDX THz spectrum. The 7 K RDX spectrum is noteworthy for several mode shifts and spectral detail that greatly aids mode assignments. Previous RDX isolated-molecule calculations (with 6 calculated modes below 125 cm^-1) are incapable of accurately predicting the numerous features in this region, highlighting the importance of solid-state theoretical methods for solid-state terahertz feature assignments.

Keywords: Cyclotrimethylenetrinitramine, RDX, solid-state density functional theory, terahertz (THz) spectroscopy

www.sciencedirect.com/science/journal/00092614
en.wikipedia.org/wiki/RDX
en.wikipedia.org/wiki/Terahertz
www.icpostdoc.org
people.web.psi.ch/delley/dmol3.html
www.pssrc.org/index.php?id=axel_zeitler
www.teraview.com
www.syr.edu
chemistry.syr.edu
www.syracuse.com
www.nanorex.com
www.cheng.cam.ac.uk
www.cam.ac.uk
www.teraview.com