IR Line Research Articles

Line shape analysis of two-dimensional infrared spectra.

Ultrafast two-dimensional infrared (2D IR) spectroscopy probes femtosecond to picosecond time scale dynamics ranging from solvation to protein motions. The frequency-frequency correlation function (FFCF) is the quantitative measure of the spectral diffusion that reports those dynamics and, within certain approximations, can be extracted directly from 2D IR line shapes. A variety of methods have been developed to extract the FFCF from 2D IR spectra, which, in principle, should give the same FFCF parameters, but the complexity of real experimental systems will affect the results of these analyses differently. Here, we compare five common analysis methods using both simulated and experimental 2D IR spectra to understand the effects of apodization, anharmonicity, phasing errors, and finite signal-to-noise ratios on the results of each of these analyses. Our results show that although all of the methods can, in principle, yield the FFCF under idealized circumstances, under more realistic experimental conditions they behave quite differently, and we find that the centerline slope analysis yields the best compromise between the effects we test and is most robust to the distortions that they cause.

Shell Structure Control of PPy-Modified CuO Composite Nanoleaves for Lithium Batteries with Improved Cyclic Performance

Polypyrrole (PPy)-modified CuO nanocomposites (NCs) with various shell structures have been synthesized by controlling the polymerization time of pyrrole in the presence of leaf-like CuO nanobelts (NBs) as wire templates. The synthesized CuO/PPy NCs and CuO NBs are characterized by XRD, FT–IR, TGA, SEM, TEM, STEM, and EDX line analysis/elemental mapping. The formation mechanism of CuO/PPy core–shell NCs is also illustrated. Electrochemical lithium-storage properties of all the products are evaluated by using them as anode materials for Li-ion batteries (LIBs). It is found that the polymerization time of pyrrole plays a significant role in affecting the shell structures and subsequent lithium-storage properties of the hybrid CuO/PPy NCs. With the extension of polymerization time, CuO/PPy NCs gradually form typical core–shell structures, where the doped PPy with increasing content is steadily and uniformly coated on the CuO surface. Correspondingly, the discharge/charge capacity and cyclic durability of CuO...

THERMONUCLEAR SUPERNOVAE: PROBING MAGNETIC FIELDS BY POSITRONS AND LATE-TIME IR LINE PROFILES

We study the imprint of magnetic fields B on late-time IR line profiles and light curves of Type Ia Supernovae. As a benchmark, we use the explosion of a Chandrasekhar mass M_{Ch White Dwarf (WD) and, specifically, a delayed detonation model. We assume WDs with initial magnetic surface fields between 1 and 1E9G. We discuss large-scale dipole and small-scale magnetic fields. We find that the [Fe II] line at 1.644 mu can be used to analyze the overall chemical and density structure of the exploding WD up to day 200 without considering B. Subsequently, positron transport and magnetic field effects become important. By day 500, the profile becomes sensitive to the morphology of B and directional dependent for dipole fields. Small or no directional dependence of the spectra is found for small-scale B. After about 200 days, persistent broad-line, flat-topped or stumpy profiles require high density burning which is the signature of a WD close to M_Ch. Narrow peaked profiles are a signature of chemical mixing or sub-MCh WDs. Good time coverage is required to separate the effects of optical depth, the size and morphology of B, and the aspect angle of the observer. The spectra require a resolution of about 500 km/sec and a signal to noise ratio of about 20%. Line blending effect are demonstrated at the example of equally prominent features at about 1.5 and 1.8 mu. For some SNeIa, spectra beyond day 300 have been observed which lend support for M_Ch mass explosions in at least some cases, and require magnetic fields equal to or in excess of 1E6G. We briefly discuss the effects of the size and morphology of B on light curves and the limitations in light of the diversity of SNeIa. We argue that line profiles are a more direct measurement of B.

Exploring the relationship between black hole accretion and star formation with blind mid-/far-infrared spectroscopic surveys

We present new estimates of redshift-dependent luminosity functions of IR lines detectable by SPICA/SAFARI and excited both by star formation and by AGN activity. The new estimates improve over previous work by using updated evolutionary models and dealing in a self consistent way with emission of galaxies as a whole, including both the starburst and the AGN component. New relationships between line and AGN bolometric luminosity have been derived and those between line and IR luminosities of the starburst component have been updated. These ingredients were used to work out predictions for the source counts in 11 mid/far-IR emission lines partially or entirely excited by AGN activity. We find that the statistics of the emission line detection of galaxies as a whole is mainly determined by the star formation rate, because of the rarity of bright AGNs. We also find that the slope of the line integral number counts is flatter than 2 implying that the number of detections at fixed observing time increases more by extending the survey area than by going deeper. We thus propose a wide spectroscopic survey of 1 hour integration per field-of-view over an area of 5 deg$^{2}$ to detect (at 5$\sigma$) $\sim$760 AGNs in [OIV]25.89$\mu$m $-$ the brightest AGN mid-infrared line $-$ out to $z\sim2$. Pointed observations of strongly lensed or hyper-luminous galaxies previously detected by large area surveys such as those by Herschel and by the SPT can provide key information on the galaxy-AGN co-evolution out to higher redshifts.

Crystal structure, vibrational and optical properties of N-(3-ammoniumpropyl)-1,3 diammoniumpropane hexabromobismuthate monohydrate: [C6H20N3]BiBr6 H2O

The chemical preparation, the main structural, vibrational and optical features are given for a new zero dimensional alkyl tri-ammonium bismuth halogenide. (C6H20N3)BiBr6·H2O crystallize in triclinic system, space group P1¯. The crystal structure consists of discrete ionic pairs of N-(3-ammoniumpropyl)-1,3 diammoniumpropane cations, hexabromobismuthate anions and neutral water molecules linked via simple and bifurcated N–H⋯Br(OW) and O(W)–H⋯Br hydrogen bonds. Infrared and Raman spectra of (C6H20N3)BiBr6 H2O were recorded at 298K and discussed. The assignment of the observed IR and Raman lines was performed by comparison with the homologous compounds. Optical properties are investigated by optical absorption UV–visible and photoluminescence (PL) techniques. The optical absorption spectra reveal the appearance of sharp optical gaps of 3.10eV as well as two strong green–yellow photoluminescence emissions are observed at room temperature.

Observations and NLTE modeling of Ellerman bombs

Ellerman bombs (EBs) are short-lived and compact structures that are observed well in the wings of the hydrogen H-alpha line. EBs are also observed in the chromospheric CaII lines and in UV continua. H-alpha line profiles of EBs show a deep absorption at the line center and enhanced emission in the line wings. Similar shapes of the line profiles are observed for the CaII IR line at 8542 ang. It is generally accepted that EBs may be considered as compact microflares located in lower solar atmosphere. However, it is still not clear where exactly the emission of EBs is formed in the solar atmosphere. High-resolution spectrophotometric observations of EBs were used for determining of their physical parameters and construction of semi-empirical models. In our analysis we used observations of EBs obtained in the H-alpha and CaII H lines. We also used NLTE numerical codes for the construction of grids of 243 semi-empirical models simulating EBs structures. In this way, the observed emission could be compared with the calculated line spectra. For a specific model we found reasonable agreement between the observed and theoretical emission and thus we consider such model as a good approximation of the EBs atmospheres. This model is characterized by an enhanced temperature in the lower chromosphere and can be considered as a compact structure (hot spot). For the first time the set of two lines H-alpha and CaII H was used to construct semi-empirical models of EBs. Our analysis shows that EBs can be described by a "hot spot" model, with the temperature and/or density increase through a few hundred km atmospheric structure. We confirmed that EBs are located close to the temperature minimum or in the lower chromosphere. Two spectral features, observed simultaneously, significantly strengthen the constraints on a realistic model.

A THREE-DIMENSIONAL VIEW OF THE THERMAL STRUCTURE IN A SUPER-PENUMBRAL CANOPY

We investigate the thermal topology in a super-penumbral canopy by determining the 3D thermal structure of an active region. We derive the temperature stratifications in the active region by an inversion of the Ca II IR line at 854.2 nm, assuming LTE. We trace the 3D topology of individual features located in the super-penumbral canopy, mainly radially oriented fibrils. We find that about half of the fibrils form short, arched, low-lying loops in the temperature cube. These closed loops connect from bright grains that are either in or close to the penumbra to the photosphere a few Mms away from the sunspot. They reach less than 1 Mm in height. The other half of the fibrils rise with distance from the sunspot until they leave the Ca II IR formation height. Many of the fibrils show a central dark core and two lateral brightenings as seen in line-core intensity images. The corresponding velocity image shows fibrils that are as wide as the fibrils seen in intensity without a lateral substructure. Additionally, we study one example of exceptional brightness in more detail. It belongs to a different class of structures without prominent mass flows and with a 3D topology formed by two parallel, closed loops connecting patches of opposite polarity. We present evidence that the inverse Evershed flow into the sunspot in the lower chromosphere is the consequence of siphon flows along short loops that connect photospheric foot points. The dark-cored structure of the chromospheric fibrils cannot have an convective origin because of their location above regular granulation in an optically thin atmosphere. The dark core most likely results from an opacity difference between the central axis and the lateral edges caused by the significant flow speed along the fibrils.

The HH34 outflow as seen in [Fe ii] 1.64μm by LBT-LUCI

Dense atomic jets from young stars copiously emit in [FeII] IR lines, which can, therefore, be used to trace the immediate environments of embedded protostars. We want to investigate the morphology of the bright [FeII] 1.64um line in the jet of the source HH34 IRS and compare it with the most commonly used optical tracer [SII]. We analyse a 1.64um narrow-band filter image obtained with the Large Binocular Telescope (LBT) LUCI instrument, which covers the HH34 jet and counterjet. A Point Spread Function (PSF) deconvolution algorithm was applied to enhance spatial resolution and make the IR image directly comparable to a [SII] HST image of the same source. The [FeII] emission is detected from both the jet, the (weak) counter-jet, and from the HH34-S and HH34-N bow shocks. The deconvolved image allows us to resolve jet knots close to about 1\arcsec from the central source. The morphology of the [FeII] emission is remarkably similar to that of the [SII] emission, and the relative positions of [FeII] and [SII] peaks are shifted according to proper motion measurements, which were previously derived from HST images. An analysis of the [FeII]/[SII] emission ratio shows that Fe gas abundance is much lower than the solar value with up to 90% of Fe depletion in the inner jet knots. This confirms previous findings on dusty jets, where shocks are not efficient enough to remove refractory species from grains.

English

The objective of the present work was to isolate and characterize the exopolymer producer from a marine environment. The exopolymer producing strain (S13) was identified as Bacillus thuringiensis S13. Characterization of exopolymer shows the presence of brominated compound responsible for cytotoxicity on lung cancer cell line (A549) on XTT assay. An in vitro study of bacterial exopolymer shows the presence of cytotoxic effects on cell lines. Further, active compound in exopolymer responsible for cytotoxicity has to be characterized. The exopolymer produced by B. thuringiensis S13, showed potent cytotoxic effects, and could be used as therapy in cancer after further studies. Key words: Bacillus thuringiensis S13, exopolymer, IR, GCMS analysis and lung cancer cell line.

SiO masers from AGB stars in the vibrationally excitedv= 1,v= 2, andv= 3 states

The v=1 and v=2 J=1-0 (43 GHz), and v=1 J=2-1 (86 GHz) SiO masers are intense in AGB stars and have been mapped using VLBI showing ring-like distributions. Those of the v=1, v=2 J=1-0 masers are similar, but the spots are rarely coincident, while the v=1 J=2-1 maser arises from a well separated region farther out. These relative locations can be explained by models tools that include the overlap of two IR lines of SiO and H2O. The v=3 J=1-0 line is not directly affected by any line overlap and its spot structure and position, relative to the other lines, is a good test to the standard pumping models. We present single-dish and simultaneous VLBI observations of the v=1, v=2, and v=3 J=1-0 maser transitions of 28SiO in several AGB stars. The spatial distribution of the SiO maser emission in the v=3 J=1-0 transition from AGB stars is systematically composed of a series of spots that occupy a ring-like structure. The overall ring structure is extremely similar to that found in the other 43 GHz transitions and is very different from the structure of the v=1 J=2-1 maser. The positions of the individual spots of the different 43 GHz lines are, however, very rarely coincident, which in general is separated by about 0.3 AU (between 1 and 5 mas). These results are very difficult to reconcile with standard pumping models, which predict that the masers of rotational transitions within a given vibrational state require very similar excitation conditions, while the transitions of different vibrational states should appear in different positions. However, models including line overlap tend to predict v=1, v=2, v=3 J=1-0 population inversion to occur under very similar conditions, while the requirements for v=1 J=2-1 appear clearly different, and are compatible with the observational results.

Ubiquitin‐conjugating enzyme UbcD4 as an essential component of Drosophila immune deficiency pathway (LB192)

Ubiquitination has been emerged as a key regulatory mechanism in the NF‐κB pathway, in which the different types of ubiquitin chains are involved in the discrete steps. Given that the ubiquitin conjugating enzymes (E2s) are key enzymes to determine the chain topologies during ubiquitination, we tried to systematically screen out the E2s required for the IMD pathway in Drosophila, which is very similar to TNF‐α mediated NF‐κB pathway in mammals, via RNAi‐mediated knockdown of individual E2 using GAL4/UAS system. For this purpose, we first generated a transgenic fly containing a reporter dipt‐GPF gene and a fat body driver hml‐Gal4 gene that expressed the reporter protein GFP upon challenge of Gram‐negative bacteria, Ecc15. When dipt‐GFP, hml‐Gal4 transgenic flies were crossed to each of 35 UAS‐E2‐RNAi IR lines obtained from VDRC and the offspring female adult flies were infected with Ecc15, and the expressed GFP in each IR offspring was compared by Western immunoblot, it was found that the GFP level was significantly reduced in IR lines targeting UbcD4 in addition to effete and bendless. RNAi‐mediated down‐regulation of a mammalian UbcD4 homolog, Ube2K/E2‐25K, significantly attenuated the TNF‐α mediated NF‐κB activation pathway. Taken together, UbcD4 and Ube2K/E2‐25K play a conserved role as a positive regulator in Drosophila IMD pathway and mammalian TNF‐α mediated NF‐κB pathway.

An experimental and theoretical study of nitrogen-broadened acetylene lines

We present experimental nitrogen-broadening coefficients derived from Voigt profiles of isotropic Raman Q-lines measured in the ν2 band of acetylene (C2H2) at 150K and 298K, and compare them to theoretical values obtained through calculations that were carried out specifically for this work. Namely, full classical calculations based on Gordon׳s approach, two kinds of semi-classical calculations based on Robert–Bonamy method as well as full quantum dynamical calculations were performed. All the computations employed exactly the same ab initio potential energy surface for the C2H2–N2 system which is, to our knowledge, the most realistic, accurate and up-to-date one. The resulting calculated collisional half-widths are in good agreement with the experimental ones only for the full classical and quantum dynamical methods. In addition, we have performed similar calculations for IR absorption lines and compared the results to bibliographic values. Results obtained with the full classical method are again in good agreement with the available room temperature experimental data. The quantum dynamical close-coupling calculations are too time consuming to provide a complete set of values and therefore have been performed only for the R(0) line of C2H2. The broadening coefficient obtained for this line at 173K and 297K also compares quite well with the available experimental data. The traditional Robert–Bonamy semi-classical formalism, however, strongly overestimates the values of half-width for both Q- and R-lines. The refined semi-classical Robert–Bonamy method, first proposed for the calculations of pressure broadening coefficients of isotropic Raman lines, is also used for IR lines. By using this improved model that takes into account effects from line coupling, the calculated semi-classical widths are significantly reduced and closer to the measured ones.

PH-Dependent Picosecond Structural Dynamics in the Distal Pocket of Nitrophorin 4 Investigated by 2D IR Spectroscopy

Nitrophorin 4 (NP4) belongs to a family of pH-sensitive, nitric oxide (NO) transporter proteins that undergo a large structural change from a closed to an open conformation at high pH to allow for NO delivery. Measuring the pH-dependent structural dynamics in NP4-NO around the ligand binding site is crucial for developing a mechanistic understanding of NO binding and release. In this study, we use coherent two-dimensional infrared (2D IR) spectroscopy to measure picosecond structural dynamics sampled by the nitrosyl stretch in NP4-NO as a function of pH at room temperature. Our results show that both the closed and open conformers of the protein are present at low (pD 5.1) and high (pD 7.9) pH conditions. The closed and open conformers are characterized by two frequencies of the nitrosyl stretching vibration labeled A0 and A1, respectively. Analysis of the 2D IR line shapes reveals that at pD 5.1, the closed conformer experiences structural fluctuations arising from solvation dynamics on a ∼3 ps time scale. At pD 7.9, both the open and closed conformers exhibit fluctuations on a ∼1 ps time scale. At both pD conditions, the closed conformers maintain a static distribution of structures within the experimental time window of 100 ps. This is in contrast to the open conformer, which is able to interconvert among its substates on a ∼100 ps time scale. Our results directly measure the time scales of solvation dynamics in the distal pocket, the flexibility of the open conformation at high pH, and the rigidity of the closed conformers at both pH conditions. We discuss how the pH-dependent equilibrium structural fluctuations of the nitrosyl ligand measured in this study are related to the uptake and delivery of nitric oxide in NP4.

Near-infrared spectroscopy of SN 2009ip's 2012 brightening reveals a dusty pre-supernova environment

Abstract We present low-resolution near-infrared (IR) 0.8–2.5 μm spectra of Supernova (SN) 2009ip, taken immediately before, during and just after its rapid brightening in late September/October 2012. The first epoch shows the same general spectral characteristics as the later epochs (smooth continuum, narrow H and He i emission lines), but the IR continuum shape is substantially redder than the later epochs. The epoch 1 continuum can be approximated by reddening the peak-luminosity (epoch 3) spectrum by E(B − V) = 1.0 mag, but the blue colour seen in visual-wavelength spectra at the same time indicates that strong wavelength-dependent extinction by circumstellar dust is not the correct explanation. Instead, we favour the hypothesis that the redder colour before the brightening arises from excess emission from hot ∼2000 K circumstellar dust. The minimum radius ( ≳ 120 au) deduced from the dust temperature and observed luminosity of the transient, combined with the observed expansion speed in the precursor outbursts of SN 2009ip, is consistent with an ejection at least 1.1 yr earlier. The mass of hot dust indicated by the IR excess is ∼4 × 10−7 M⊙, although this is only a lower limit since the near-IR data do not constrain the mass of cooler dust. Thus, the observed pre-SN outbursts of this object were able to efficiently form dust into which the SN ejecta and radiation now propagate. This is consistent with the notion that the same pre-SN eruptions that generally give rise to SNe IIn also give rise to the dust needed for their commonly observed IR echoes. We also discuss some aspects of the IR line profiles, including He i λ10 830.

H2 annealing effect on the structural and electrical properties of amorphous InGaZnO films for thin film transistors

Amorphous indium–gallium–zinc oxide (a-IGZO) thin films were deposited on quartz glass substrates via radio frequency magnetron sputtering. The effect of H2 annealing on the structural, electrical, and H-associated vibrational absorption properties of the a-IGZO films has been systematically studied. The temperature dependence of electrical conductivity of the as-grown and H2 annealed films showed n-type degenerated conduction behavior in the range of 10–300K. Transport properties of these a-IGZO films do not fit well with variable range hopping model. Carrier mobility exhibited increased thermally-exited behavior as H2 annealing temperature is higher than 300°C, suggesting existence of potential barriers above the mobility edge. Fourier Transform Infrared Spectroscopy illustrated OH vibrational mode absorption at around 3670cm−1, suggesting the formation of OH bonding by H2 annealing process which increased at 250°C then annealed away above 300°C along with electron carriers in the a-IGZO films. Comparatively, this OH ir line was greatly mitigated in the a-IGZO films annealed in air. Decrease in electron concentration results from reduced OH absorption in the H2 annealed a-IGZO films upon annealing above 300°C. Surface roughness of the a-IGZO films decreased with increasing H2 annealing temperature for the improvement of device interface uniformity.

A new treatment of the v4 = 2 excited state of the symmetric top molecule PF3 around 693 cm−1

All experimental data of the 2ν40 parallel and 2ν4-2 perpendicular components of the pyramidal molecule PF3 have been refined using five equivalent D-, Q-, QD-, L-, LD-reduction forms of the effective rovibrational Hamiltonian recently developed for the vt(E)=2 vibrational state of a C3v symmetric top molecule. The v4=2 excited level of the PF3 molecule has been treated with models taking into account ℓ- and k-intravibrational resonances. The body of data comprised 1171 IR lines of the 2ν40 component, 249 energies of the v4=2−2 substate deduced from the 2ν4-2-ν4-1 hot band and 5 reported MW data. The standard deviations of the fits are practically similar for the reductions applied and close to the quality of measurements. The unitary equivalence of the tested relations between the derived parameters was satisfactorily fulfilled.

Effect of Solvent Polarity on the Vibrational Dephasing Dynamics of the Nitrosyl Stretch in an FeII Complex Revealed by 2D IR Spectroscopy

The vibrational dephasing dynamics of the nitrosyl stretching vibration (ν(NO)) in sodium nitroprusside (SNP, Na2[Fe(CN)5NO]·2H2O) are investigated using two-dimensional infrared (2D IR) spectroscopy. The ν(NO) in SNP acts as a model system for the nitrosyl ligand found in metalloproteins which play an important role in the transportation and detection of nitric oxide (NO) in biological systems. We perform a 2D IR line shape study of the ν(NO) in the following solvents: water, deuterium oxide, methanol, ethanol, ethylene glycol, formamide, and dimethyl sulfoxide. The frequency of the ν(NO) exhibits a large vibrational solvatochromic shift of 52 cm(-1), ranging from 1884 cm(-1) in dimethyl sulfoxide to 1936 cm(-1) in water. The vibrational anharmonicity of the ν(NO) varies from 21 to 28 cm(-1) in the solvents used in this study. The frequency-frequency correlation functions (FFCFs) of the ν(NO) in SNP in each of the seven solvents are obtained by fitting the experimentally obtained 2D IR spectra using nonlinear response theory. The fits to the 2D IR line shape reveal that the spectral diffusion time scale of the ν(NO) in SNP varies from 0.8 to 4 ps and is negatively correlated with the empirical solvent polarity scales. We compare our results with the experimentally determined FFCFs of other charged vibrational probes in polar solvents and in the active sites of heme proteins. Our results suggest that the vibrational dephasing dynamics of the ν(NO) in SNP reflect the fluctuations of the nonhomogeneous electric field created by the polar solvents around the nitrosyl and cyanide ligands. The solute solvent interactions occurring at the trans-CN ligand are sensed through the π-back-bonding network along the Fe-NO bond in SNP.

A theoretical model for polarized infrared spectra of crystals of 2-naphthyl acetic acid in the OH-stretching region

This paper presents results of a theoretical study on νO-H(D) band shapes in the polarized infrared spectra of 2-naphthylacetic acid (2-NA) crystals measured at the temperature of liquid nitrogen. The line shapes were studied within the frameworks of strong anharmonic coupling, Davydov coupling, Fermi resonance coupling, direct and indirect damping, and a selection rule breaking mechanism for forbidden transitions in IR. The present approach correctly fits the experimental line shape of the hydrogenated compound and predicts satisfactorily the evolution in the line shapes with isotopic substitution. Numerical calculations show that mixing of all these effects allows one to reproduce satisfactorily the main features of the experimental IR line shapes of hydrogenated and deuterated 2-NA crystals and is expected to confirm the importance of the Fermi resonances in reproducing the experimental spectra.

Ligand binding studied by 2D IR spectroscopy using the azidohomoalanine label.

We explore the capability of the azidohomoalanine (Aha) as a vibrational label for 2D IR spectroscopy to study the binding of the target peptide to the PDZ2 domain. The Aha label responds sensitively to its local environment and its peak extinction coefficient of 350-400 M(-1) cm(-1) is high enough to routinely measure it in the low millimolar concentration regime. The central frequency, inhomogeneous width and spectral diffusion times deduced from the 2D IR line shapes of the Aha label at various positions in the peptide sequence is discussed in relationship to the known X-ray structure of the peptide bound to the PDZ2 domain. The results suggest that the Aha label introduces only a small perturbation to the overall structure of the peptide in the binding pocket. Finally, Aha is a methionine analog that can be incorporated also into larger proteins at essentially any position using protein expression. Altogether, Aha thus fulfills the requirements a versatile label should have for studies of protein structure and dynamics by 2D IR spectroscopy.

Small polaron characteristics of an OD center in TiO2studied by infrared spectroscopy

The fundamental OD center in TiO${}_{2}$ has been studied by IR absorption spectroscopy and by theory. Our experiments reveal a single sharp OD line in oxygenated samples that we attribute to a positively charged center. In TiO${}_{2}$ samples reduced by a treatment in a D${}_{2}$ ambient at 500 \ifmmode^\circ\else\textdegree\fi{}C, a neutral OD center with two additional IR lines is produced. Similar results reported recently by Herklotz et al. [F. Herklotz, E. V. Lavrov, and J. Weber, Phys. Rev. B 83, 235202 (2011)] led them to suggest that the neutral OD center is a shallow donor with IR lines arising from its two charge states. Our analysis suggests instead that the properties of the neutral OD center are explained by a small polaron model, which involves several configurations corresponding to the localization of the electron on different nearby Ti sites.