Modeling red coral (Corallium rubrum) and African snail (Helixia aspersa) shell pigments: Raman spectroscopy versus DFT studies

Pigments from red coral (Corallium rubrum) and African snail (Helixia aspersa) shell were studied non‐invasively using Raman spectroscopy with 1064‐nm laser beam. The two observed bands because of organic pigments confined in biomineralized CaCO₃ matrix at about 1500 and 1100 cm⁻¹ were assigned to ν(CC) and ν(C―C), respectively. Both signals originate from polyene(s) of largely unknown structure, containing several conjugated CC bonds. The small peak at 1016 cm⁻¹ in the Raman spectrum of coral pigment was assigned to in‐plane ―CH₃ rocking or structural deformation of polyene chain because of spatial confinement in the mineral matrix. The organic pigments in red coral and snail shell were present in inorganic matrix containing aragonite (shell) and calcite (coral). In addition, using Raman spectroscopy, it was observed that aragonite was replaced by calcite as result of healing damaged parts of snail shell. This is an important finding which indicates a great potential of nondestructive Raman spectroscopy instead of X‐ray technique, as a diagnostic tool in environmental studies. To support analysis of the observed Raman spectra detailed calculations using density functional theory (DFT with B3LYP and BLYP density functionals) on structure and vibrations of model all‐trans polyenes were undertaken. DFT calculated CC and C―C stretching frequencies for all‐trans polyenes containing from 2 to 14 CC units were compared with the observed ν(CC) and ν(C―C) band positions of the studied coral and shell. Individual correction factors were used to better match theoretical wavenumbers with observed band positions in red coral and African snail. It was concluded that all‐trans polyene pigments of red coral and dark parts of African snail shell contain 11–12 and 14 CC double bond units, respectively. However, Raman spectroscopy cannot produce any clear information on the presence and nature of the end‐chain substituents in the studied pigments. Copyright © 2016 John Wiley & Sons, Ltd.     

Determination by Raman scattering of the nature of pigments in cultured freshwater pearls from the mollusk Hyriopsis cumingi

The nature of pigments in naturally colored pearls is still under discussion. For this study, Raman scattering measurements were obtained for 30 untreated freshwater cultured pearls from the mollusk Hyriopsis cumingi covering their typical range of colors. The originality of this work is that seven different excitation wavelengths (1064 nm, 676.44 nm, 647.14 nm, 514.53 nm, 487.98 nm, 457.94 nm, 363.80 nm) are used for the same samples at the highest possible resolution. All colored pearls show the two major Raman features of polyenic compounds assigned to double carbon–carbon (CC) – at about 1500 cm⁻¹ – and single carbon–carbon (C C) – at about 1130 cm⁻¹ – bond stretching mode, regardless of their specific hue. These peaks are not detected in the corresponding white pearls, and therefore seem directly related to the major cause of body color. Additionally, the exact position of CC stretching vibration shows that these compounds are not members of the carotenoid family. Moreover, some changes are observed in intensities, shape and positions of the two main characteristic polyenic peaks from one sample to the next. Similar changes are observed also using several excitation wavelengths for the same point of the same pearl. The exact position of C C stretching vibration of polyenic molecules depends strongly on the number of double bonds (N) contained in their polyenic chain. Hence, using a constrained decomposition of this band for different excitation wavelengths, up to nine different pigments may be detected in the same pearl. Their general chemical formula is R‐( CHCH )_N‐R′ with N = 6–14. All our colored samples contained at least four pigments (N = 8–11). Different colors are explained by different mixtures, not by a simple change of pigment. The chemical nature of the chain ends is still unknown, because it cannot be detected with Raman scattering. However, it is possible that these polyenes are complexed with carbonate molecules of the nacre. Similar coloration mechanisms are found in products from other living organisms (e.g. parrots feathers). Moreover, it seems that a similar series of pigments is found in other pearls also, as well as in some marine animals living in similar environments (e.g. corals). Copyright © 2006 John Wiley & Sons, Ltd.     

Experimental and ab initio DFT calculated Raman spectrum of Sudan I,a red dye

The red dye Sudan I was investigated by Raman spectroscopy using different excitation wavelengths (1064, 532 and 244 nm). A calculation of the Raman spectrum based on quantum mechanical ab initio density functional theory (DFT) was made using the RB3LYP method with the 3‐21G and 6‐311 + G(d,p) basis sets. The vibrations in the region 1600–1000 cm⁻¹ were found to comprise various mixed modes including in‐plane stretching and bending of various C C, N N, C N and C O bonds and angles in the molecule. Below ∼900 cm⁻¹, the out‐of‐plane bending modes were dominant. The central hydrazo chromophore of the Sudan I molecule was involved in the majority of the vibrations through NN and C N stretching and various bending modes. Low‐intensity bands in the lower wavenumber range (at about 721, 616, 463 and 218 cm⁻¹) were selectively enhanced by the resonance Raman effect when using the 532 nm excitation line. Comparison was made with other azo dyes in the literature on natural, abundant plant pigments. The results show that there is a possibility in foodstuff analysis to distinguish Sudan I from other dyes by using Raman spectroscopy with more than one laser wavelength for resonance enhancement of the different bands Copyright © 2011 John Wiley & Sons, Ltd.     

Vibrational spectroscopy and crystal structure analysis of two polymorphs of the di‐amino acid peptide cyclo(L‐Glu‐L‐Glu)

Cyclo(L ‐Glu‐L ‐Glu) has been crystallised in two different polymorphic forms. Both polymorphs are monoclinic, but form 1 is in space group P2₁ and form 2 is in space group C2. Raman scattering and FT‐IR spectroscopic studies have been conducted for the N,O‐protonated and deuterated derivatives. Raman spectra of orientated single crystals, solid‐state and aqueous solution samples have also been recorded. The different hydrogen‐bonding patterns for the two polymorphs have the greatest effect on vibrational modes with N H and CO stretching character. DFT (B3‐LYP/cc‐pVDZ) calculations of the isolated cyclo(L ‐Glu‐L ‐Glu) molecule predict that the minimum energy structure, assuming C₂ symmetry, has a boat conformation for the diketopiperazine ring with the two L ‐Glu side chains being folded above the ring. The calculated geometry is in good agreement with the X‐ray crystallographic structures for both polymorphs. Normal coordinate analysis has facilitated the band assignments for the experimental vibrational spectra. Copyright © 2009 John Wiley & Sons, Ltd.     

Crystal → nematic phase transition in the liquid crystalline system 1‐isothiocyanato‐4‐(trans‐4‐propylcyclohexyl) benzene (3CHBT) probed by temperature‐dependent micro‐Raman study and DFT calculations

Raman spectra of 3CHBT in unoriented form were recorded at 14 different temperature measurements in the range 25–55 °C, which covers the crystal → nematic (N) phase transition, and the Raman signatures of the phase transition were identified. The wavenumber shifts and linewidth changes of Raman marker bands with varying temperature were determined. The assignments of important vibrational modes of 3CHBT were also made using the experimentally observed Raman and infrared spectra, calculated wavenumbers, and potential energy distribution. The DFT calculations using the B3LYP method employing 6‐31G functional were performed for geometry optimization and vibrational spectra of monomer and dimer of 3CHBT. The analysis of the vibrational bands, especially the variation of their peak position as a function of temperature in two different spectral regions, 1150–1275 cm⁻¹ and 1950–2300 cm⁻¹, is discussed in detail. Both the linewidth and peak position of the ( C H ) in‐plane bending and ν(NCS) modes, which give Raman signatures of the crystal → N phase transition, are discussed in detail. The molecular dynamics of this transition has also been discussed. We propose the co‐existence of two types of dimers, one in parallel and the other in antiparallel arrangement, while going to the nematic phase. The structure of the nematic phase in bulk has also been proposed in terms of these dimers. The red shift of the ν(NCS) band and blue shift of almost all other ring modes show increased intermolecular interaction between the aromatic rings and decreased intermolecular interaction between two  NCS groups in the nematic phase. Copyright © 2009 John Wiley & Sons, Ltd.     

Vibrational spectroscopic studies of the structure of di‐amino acid peptides. Part II: cyclo(L‐Asp‐L‐Asp) in the solid state and in aqueous solution

Solid‐state protonated and N,O‐deuterated Fourier transform infrared (IR) and Raman scattering spectra together with the protonated and deuterated Raman spectra in aqueous solution of the cyclic di‐amino acid peptide cyclo(L ‐Asp‐L ‐Asp) are reported. Vibrational band assignments have been made on the basis of comparisons with previously cited literature values for diketopiperazine (DKP) derivatives and normal coordinate analyses for both the protonated and deuterated species based upon DFT calculations at the B3‐LYP/cc‐pVDZ level of the isolated molecule in the gas phase. The calculated minimum energy structure for cyclo(L ‐Asp‐L ‐Asp), assuming C₂ symmetry, predicts a boat conformation for the DKP ring with both the two L ‐aspartyl side chains being folded slightly above the ring. The CO stretching vibrations have been assigned for the side‐chain carboxylic acid group (e.g. at 1693 and 1670 cm⁻¹ in the Raman spectrum) and the cis amide I bands (e.g. at 1660 cm⁻¹ in the Raman spectrum). The presence of two bands for the carboxylic acid CO stretching modes in the solid‐state Raman spectrum can be accounted for by factor group splitting of the two nonequivalent molecules in a crystallographic unit cell. The cis amide II band is observed at 1489 cm⁻¹ in the solid‐state Raman spectrum, which is in agreement with results for cyclic di‐amino acid peptide molecules examined previously in the solid state, where the DKP ring adopts a boat conformation. Additionally, it also appears that as the molecular mass of the substituent on the C^α atom is increased, the amide II band wavenumber decreases to below 1500 cm⁻¹; this may be a consequence of increased strain on the DKP ring. The cis amide II Raman band is characterized by its relatively small deuterium shift (29 cm⁻¹), which indicates that this band has a smaller N H bending contribution than the trans amide II vibrational band observed for linear peptides. Copyright © 2009 John Wiley & Sons, Ltd.     

IR,Raman and SERS spectra of 5‐sulphosalicylic acid dihydrate

IR, Raman and surface‐enhanced Raman scattering (SERS) spectra of 5‐sulphosalicylic acid were recorded and analysed. The vibrational wavenumbers were computed by density functional theoretical (DFT) method using B3LYP/6–31G* basis. The bands due to the stretching modes CO, C S and SO₂ are intense in the SERS spectrum. The C H stretching mode also appears in the SERS spectrum. The molecule is found to adsorb through both the carboxyl and sulphonyl groups. A possible tilted orientation of the molecule is suggested. Copyright © 2006 John Wiley & Sons, Ltd.     

Experimental vibrational spectra (Raman,infrared) and DFT calculations on monomeric and dimeric structures of 2‐ and 6‐bromonicotinic acid

In this work, the Fourier transform infrared and Raman spectra of 2‐bromonicotinic acid and 6‐bromonicotinic acid (abbreviated as 2‐BrNA and 6‐BrNA, C₆H₄BrNO₂) have been recorded in the region 4000–400 and 3500–50 cm⁻¹. The optimum molecular geometry, normal mode wavenumbers, infrared intensities and Raman scattering activities, corresponding vibrational assignments and intermolecular hydrogen bonds were investigated with the help of B3LYP density functional theory (DFT) method using 6‐311++G(d,p) basis set. Reliable vibrational assignments were made on the basis of total energy distribution (TED) calculated with scaled quantum mechanical (SQM) method. From the calculations, the molecules are predicted to exist predominantly as the C1 conformer. Copyright © 2009 John Wiley & Sons, Ltd.     

Effects of conformation and hydrogen bonding on the CC and NCN stretching Raman bands of N‐deuterated histidinium

Histidine is an important and versatile amino acid residue that plays a variety of structural and functional roles in proteins. Although the Raman bands of histidine are generally weak, histidine in the N‐deuterated cationic form with imidazole N_π D and N_τ D bonds (N‐deuterated histidinium) gives two strong Raman bands assignable to the C₄C₅ stretch (ν_CC) and the N_π C₂ N_τ symmetric stretch (ν_NCN) of the imidazole ring. We examined the Raman spectra of N‐deuterated histidinium in 12 crystals with known structures. The observed ν_CC and ν_NCN wavenumbers were analyzed to find empirical correlations with the conformation and hydrogen bonding. The effect of conformation on the vibrational wavenumber was expressed as a threefold cosine function of the C_α C_β C₄C₅ torsional angle. The effect of hydrogen bonding at N_π or N_τ was assumed to be proportional to the inverse sixth power of the distance between the hydrogen and acceptor atoms. Multiple linear regression analysis clearly shows that the conformational effect on the vibrational wavenumber is comparable for ν_CC and ν_NCN. The hydrogen bond at N_π weakly lowers the ν_CC wavenumber and substantially raises the ν_NCN wavenumber. On the other hand, the hydrogen bond at N_τ strongly raises the ν_CC wavenumber but does not affect the ν_NCN wavenumber. These empirical correlations may be useful in Raman spectral analysis of the conformation and hydrogen bonding states of histidine residues in proteins. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis,molecular structure,IR and Raman spectra as well as DFT chemical calculations for alkylamides of thiocyanoacetic acid

This paper reports on room‐temperature infrared (IR) and Raman studies and vibrational characteristics of amide and thiocyano groups of R‐NH‐CO‐CH₂‐SCN n‐alkylamides of thiocyanoacetic acid (R = C₈H₁₇, C₉H₁₉, C₁₂H₂₅ and C₁₄H₂₉). Their molecular structure has been proposed on the basis of optimization process. The experimental wavenumbers have been compared to those obtained from discrete Fourier transform (DFT) quantum chemical calculations performed with the use of B3LYP/6–31G(d,p) approximation. The role of the hydrogen bonds in the stabilization of the structure has been analyzed. It was found that the hydrogen bonding and strong dynamic interactions between the unit cell components are responsible for the deviation of several theoretical wavenumbers from the experimental ones. Copyright © 2009 John Wiley & Sons, Ltd.     

Vibrational spectroscopic study of budesonide

The Raman spectrum of budesonide is reported for the first time, and molecular assignments are proposed on the basis of ab initio BLYP DFT calculations with a 6‐31 G* basis set and vibrational wavenumbers predicted on a quasi‐harmonic approximation. Comparison with previously published infrared data has explained several spectral features, and the relative band intensities in the CO and CC stretching regions are interpreted. The results from this study provide data that can be used for the preparative process monitoring of budesonide, an important steroidal pharmaceutical in various dosage forms, and its interaction with excipients and other components. Copyright © 2007 John Wiley & Sons, Ltd.     

基于拉曼光谱实验与密度泛函理论的宝石级红珊瑚拉曼谱峰归属研究

红珊瑚是一种珍贵的有机宝石,自古以来因其红润的颜色与细腻的质地深受人们的喜爱与追捧。颜色漂亮的天然红珊瑚产量稀少,故有些红珊瑚会经过染色处理来改善其外观。拉曼光谱测试是鉴定红珊瑚有无经过染色处理的有力手段,故红珊瑚拉曼谱峰的归属对于鉴定有着重要的理论指导意义。由于红珊瑚拉曼峰的归属问题一直没有被深入研究,基于此,该研究测试了三颗颜色深浅不同的红珊瑚（Corallium rubrum）的拉曼光谱。同时,使用量子化学程序Gaussian 16运用密度泛函理论计算了红珊瑚中色素分子角黄素的理论拉曼光谱。创新性对比红珊瑚的实验拉曼光谱与角黄素分子的理论拉曼光谱,并进一步分析红珊瑚拉曼峰的归属。结果发现,红珊瑚的拉曼光谱中主要有1 514,1 295,1 177,1 125,1 086和1 016 cm-1拉曼峰,其中1 086 cm-1处的拉曼峰是方解石的CO2-₃引起的。红珊瑚的红色越深,1 514,1 295,1 177,1 125和1 016 cm-1拉曼峰的强度越强,反之,红珊瑚的红色越浅,这些拉曼峰的强度越弱。红珊瑚拉曼光谱中的1 514,1 295,1 177,1 125和1 016 cm-1峰强与红珊瑚的红色深浅呈现出正相关的关系,故推测这套拉曼峰是由红珊瑚中的色素产生的。角黄素理论拉曼光谱中主要存在的拉曼峰位于1 512,1 269,1 189,1 159和999 cm-1处,与红珊瑚实验拉曼光谱中的1 514,1 295,1 177,1 125和1 016 cm-1峰的形状位置高度吻合。振动分析结果表明,角黄素的1 512,1 269,1 189,1 159和999 cm-1拉曼峰分别是由CC伸缩振动,C-H摇摆振动,C-C伸缩振动,C-C伸缩振动与甲基摇摆振动引起的。因此将红珊瑚拉曼光谱中的1 514,1 295,1 177,1 125和1 016 cm-1峰归属为CC伸缩振动,C-H摇摆振动,C-C伸缩振动,C-C伸缩振动与甲基摇摆振动。使用密度泛函理论的计算方法研究了红珊瑚拉曼谱峰的归属并对红珊瑚的拉曼谱峰进行了指认,为使用拉曼光谱鉴定红珊瑚提供了理论基础。同时为研究这类生物宝石材料拉曼谱峰的归属提供了一种新的方法。     

Dynamics and mechanism of the Crystal II → smecticG phase transition in TB7A by a temperature‐dependent micro‐Raman study and DFT calculations

The solid to smecticG (SmG) phase transition in a Schiff base liquid crystalline compound, terepthal‐bis‐heptylaniline (TB7A), is monitored in situ by temperature‐dependent Raman microspectroscopy, using the band of a C H in‐plane bending mode as a marker. Contrary to the earlier report of a sudden wavenumber shift, the in situ measurement shows very clearly that a new Raman band at ∼1160 cm⁻¹ appears at the Crystal II → SmG transition. The dynamics of this phase transition is discussed in terms of a triple well potential below 210 K and a double well potential above 210 K. The phase transition essentially takes place as a result of intra‐molecular rotation about the long molecular axis. The optimization energy at various fixed dihedral angles, ( C C CN ) are calculated using density functional theory (DFT) at the B3LYP/6‐31G* level of theory. The relative energy at each dihedral angle is calculated relative to optimization energy obtained without any constraints and plotted as a function of dihedral angle (Φ) between the adjacent phenyl ring planes, which also shows a double well potential at room temperature. Copyright © 2009 John Wiley & Sons, Ltd.     

Vibrational properties of levulinic acid and furan derivatives: Raman spectroscopy and theoretical calculations

In this work, the Raman spectra of furan, furfuryl alcohol (FA), furfural, hydroxymethylfurfural (HMF), and levulinic acid were obtained in the 500 to 4000 cm⁻¹ spectral region at room temperature. Vibrational wavenumbers were calculated for these compounds with the B3LYP method using the 6‐31 + G(2df,p) basis set. The experimentally determined CC and C C wavenumbers for furan and furan derivatives were in good agreement with the calculated wavenumbers without scaling factor, while the calculated CO and C H wavenumbers at ∼1660 and 3000 cm⁻¹, respectively, showed larger deviations from the measured ones. The Raman spectra for furan and furan derivatives showed intense CC bands, whereas the levulinic acid spectrum showed intense C H vibrations with broad doublet CO bands. We also found that an empirical method based on the chemical structure similarities is able to predict the HMF Raman spectrum from the combined furfural and FA spectra. Copyright © 2011 John Wiley & Sons, Ltd.     

Physical image versus structure relation. Part 14—an attempt to rationalize some acidic region 13C NMR‐pH titration shifts for tetraaza macrocycles throughout the conformational GIAO DFT computational results: a pendant‐arm cyclam case

The most probable time‐averaged conformations of three polyammonium cations H_n 2 ⁿ⁺ (n = 3–5) formed from the macrocyclic pentamine ligand ( 2 , scorpiand) [derivative of 1,4,8,11‐tetraazacyclotetradecane (cyclam)] were analyzed in order to elucidate an origin of ‘wrong‐way’ amine‐protonation shifts found in some ¹³C NMR pH‐profiles determined for the acidic H₂O/D₂O solution. These NMR trends were reproduced quite well in δ_Cs computed for multicomponent shapes of related cations, which were in turn elucidated by the best fitting experimental data to those predicted by the gauge‐independent atomic orbital (GIAO) B3LYP/6‐31G* method, including the IEF‐PCM approach. A consistent DFT methodology of the treatment of such equilibrated cationic mixtures is proposed. Moreover, a few novel ONIOM2‐GIAO B3LYP/6‐31G*:STO‐3G type supermolecular calculations were performed for a simulated presence of bulk water molecules surrounding H₅ 2 ⁵⁺. Copyright © 2009 John Wiley & Sons, Ltd.     

Identification of reddish pigments in octocorals by Raman spectroscopy

The nature of the pigments in octocorals has been investigated by Raman spectroscopy, where laser excitation at 632.8 and 1064 nm were used to characterize the colored components present in the skeleton of the exotic pink‐yellow soft coral Chromonephthea braziliensis, the reddish purple sea fan Leptogorgia punicea and the endemic deep violet red Leptogorgia violacea from the southeastern coast of Brazil. The observed positions of two major Raman bands at ca 1500 cm⁻¹ [ν(CC)] and 1130 [ν(C C)] for all specimens strongly suggest the presence of a mixtureof conjugated polyenes belonging to a class of compounds named parrodienes. The hemiketal steroidal feeding deterrent, 23‐keto‐cladiellin‐A, isolated from C. braziliensis was identified in the crude extracts by the Raman analysis using 1064 nm excitation. The observation of the most important vibrational bands of this compound can be useful in future investigations to monitor its presence in crude extracts of C. braziliensis and or other species. Copyright © 2010 John Wiley & Sons, Ltd.     

Experimental and theoretical study of the structure and vibrational spectra of valpromide,C7H15CONH2

In this work, we present results of the conformational and vibrational properties of valpromide (Vpd), an amide with antiepileptic activity, studied by IR and Raman spectroscopy at 300 and 77 K, and 300 K, respectively. Experimental data are compared against ab initio calculations performed at B3LYP level with the inclusion of solvatation effects. Experimental results, reinforced by theoretical calculations, point out that Vpd has three conformers on the potential energy surface, with different structures that can be identified in the CO and NH spectral regions. These conformers are defined by different angular arrays of the dihedral angles formed with the CO, C N and C〈 H, C C of the aliphatic chain bonds. The existence of different conformations and structures are discussed on the basis of results derived from electronic localization function (ELF) and natural orbital bond (NBO) analysis. Copyright © 2008 John Wiley & Sons, Ltd.     

Intermolecular interactions,ion solvation,and association in mixtures of 1‐n‐butyl‐3‐methylimidazolium hexafluorophosphate and γ‐butyrolactone: insights from Raman spectroscopy

By means of Raman spectroscopy coupled with density functional theory (DFT) calculations and perturbation correlation moving window two‐dimensional correlation spectroscopy intermolecular interactions were assessed in mixtures of ionic liquid (IL) 1‐n‐butyl‐3‐methylimidazolium hexafluorophosphate (BmimPF₆) with polar aprotic solvent γ‐butyrolactone (γ‐BL) over the entire range of compositions. The symmetrical P―F stretching vibration of the IL anion was found to be insensitive to the changes in mixture concentration in contrast to the CO stretching vibration of the γ‐BL and the imidazolium ring C―H stretching vibrations of the IL cation. Each of these vibrational profiles was decomposed in various spectral contributions, and their number was rationalized by the results of quantum‐chemical calculations and/or previous controversial published data. Progressive redshift of the ring C―H stretching wavenumbers was referred to pronounced solvation of the cation at the imidazolium ring site accompanied with H‐bond formation. This was especially pronounced at IL mole fraction less than 0.18. Complicated variations in the intensities of the individual contributions of the CO profile were treated as a manifestation of the changing with concentration pattern of the intermolecular interactions. The self‐association of γ‐BL molecules and distinct cation solvation as dominant intermolecular interactions at low IL content are replaced with weaker cation solvation and ion association at high concentrations of IL. Possible representative molecular structures were proposed on the basis of DFT calculations. Copyright © 2015 John Wiley & Sons, Ltd.     

Theoretical and vibrational spectral investigation of sodium salt of acenocoumarol

The FT‐IR and FT‐Raman spectra of sodium salt of 4‐hydroxy‐3[1‐(4‐nitrophenyl)‐3‐oxobutyl]‐2H‐1‐benzopyran‐2‐one (acenocoumarol sodium salt) in solid phase have been recorded and analyzed. The optimization geometry, intramolecular hydrogen bonding, and harmonic vibrational wavenumbers of acenocoumarol sodium salt have been investigated with the help of B3LYP density functional theory (DFT) methods. The infrared and Raman spectra were predicted theoretically from the calculated intensities. Natural bond orbital (NBO) analysis indicates the presence of C H···O hydrogen bonding in the molecule. The first static hyperpolarizability of the molecule has been computed. Copyright © 2009 John Wiley & Sons, Ltd.     

A comparative study on the Ge6C14 heterofullerene nanocages: a density functional survey

Ten C₁₄Ge₆ heterofullerene isomers of C₂₀ have been investigated by density functional theory (DFT) methods with Becke 3‐Parameter (Exchange), Lee, Yang and Parr (B3LYP) functional at the 6‐311 + G*, 6‐311++G** and AUG‐cc‐pVTZ levels. In contrast to identical bonds in the latter, contractions of C═C double bonds are encountered at the expense of longer C―Ge bonds in the former. Vibrational frequency analysis confirms that all of the nanocages are true minima. In contrast to the common belief, for obtaining highly silicon‐doped stable heterofullerenes, that the silicon dopants must be completely isolated from each other by means of strong C═C double bonds. Here, linking the germanium substitutions together is an applicable strategy for obtaining highly doped stable isolated heterofullerenes since it avoids weak heteroatom─heteroatom bonds. Therefore, none of the computed heterofullerenes collapses to open, to deform, or to segregate fullerenic cages. As to band gaps (ΔE_HOMO‐LUMO), and nucleus‐independent chemical shifts at cage centers (NICS (0)), C₁₄Ge₆‐2 immerges with the highest value. Hence, it is predicted to be the most stable against electronic excitation. It contains 2 Ge─Ge single bonds at the cap‐equatorial positions. On the other hand, as to zero‐point vibrational energy and heat of atomization (ΔH_at), C₁₄Ge₆‐8 appears with the lowest and highest value, respectively. It contains 6 alternating germanium atoms in the equatorial and cap positions. Thus, it is predicted to be the most thermodynamically stable. So, germanium substitution leads to a high charge distribution on the surfaces of all the isomers specially C₁₄Ge₆‐9 with +1.496 charged germanum atoms. C₁₄Ge₆ isomers seem to be a good candidate for the hydrogen storage material.