A theoretical assignment on excited‐state intramolecular proton transfer mechanism for quercetin

In the present work, we investigate a new chromophore (ie, quercetin) (Simkovitch et al J Phys Chem B 119 [2015] 10244) about its complex excited‐state intramolecular proton transfer (ESIPT) process based on density functional theory and time‐dependent density functional theory methods. On the basis of the calculation of electron density ρ( r ) and Laplacian ?²ρ( r ) at the bond critical point using atoms‐in‐molecule theory, the intramolecular hydrogen bonds (O₁‐H₂?O₅ and O₃‐H₄?O₅) have been supported to be formed in the S₀ state. Comparing the prime structural variations of quercetin involved in its 2 intramolecular hydrogen bonds, we find that these 2 hydrogen bonds should be strengthened in the S₁ state, which is a fundamental precondition for facilitating the ESIPT process. Concomitantly, infrared vibrational spectra analysis further verifies this viewpoint. In good agreement with previous experimental spectra results, we find that quercetin reveals 2 kinds of excited‐state structures (quercetin* and quercetin‐PT1*) in the S₁ state. Frontier molecular orbitals depict the nature of electronically excited state and support the ESIPT reaction. Our scanned potential energy curves according to variational O₁‐H₂ and O₃‐H₄ coordinates demonstrate that the proton transfer process should be more likely to occur in the S₁ state via hydrogen bond wire O₁‐H₂?O₅ rather than O₃‐H₄?O₅ because of the lower potential energy barrier 2.3 kcal/mol. Our present work explains previous experimental result and makes up the deficiency of mechanism in previous experiment. In the end, we make a reasonable assignment for ESIPT process of quercetin.     

The ESIPT mechanism of dibenzimidazolo diimine sensor: a detailed TDDFT study

Spectroscopic investigations on excited state proton transfer of a new dibenzimidazolo diimine sensor (DDS) were reported by Goswami et al. recently. In our present work, based on the time‐dependent density functional theory (TDDFT), the excited‐state intramolecular proton transfer (ESIPT) mechanism of DDS is studied theoretically. Our calculated results reproduced absorption and fluorescence emission spectra of the previous experiment, which verifies that the TDDFT method we adopted is reasonable and effective. The calculated dominating bond lengths and bond angles involved in hydrogen bond demonstrate that the intramolecular hydrogen bond is strengthened. In addition, the phenomenon of hydrogen bond reinforce has also been testified based on infrared vibrational spectra. Further, hydrogen bonding strengthening manifests the tendency of ESIPT process. The calculated frontier molecular orbitals further demonstrate that the excited state proton transfer is likely to occur. According to the calculated results of potential energy curves along O–H coordinate, the potential energy barrier of about 5.02 kcal/mol is discovered in the S₀ state. However, a lower potential energy barrier of 0.195 kcal/mol is found in the S₁ state, which demonstrates that the proton transfer process is more likely to happen in the S₁ state than the S₀ state. In other words, the proton transfer reaction can be facilitated based on the photo‐excitation effectively. Moreover, the phenomenon of fluorescence quenching could be explained based on the ESIPT mechanism. Copyright © 2015 John Wiley & Sons, Ltd.     

A detailed DFT/TDDFT study on excited‐state intramolecular hydrogen bonding dynamics and proton‐transfer mechanism of 2‐phenanthro[9,10‐d]oxazol‐2‐yl‐phenol

In this present work, using density functional theory and time‐dependent density functional theory methods, we theoretically study the excited‐state hydrogen bonding dynamics and the excited state intramolecular proton transfer mechanism of a new 2‐phenanthro[9,10‐d]oxazol‐2‐yl‐phenol (2PYP) system. Via exploring the reduced density gradient versus sign(λ₂(r))ρ(r), we affirm that the intramolecular hydrogen bond O1‐H2?N3 is formed in the ground state. Based on photoexcitation, comparing bond lengths, bond angles, and infrared vibrational spectra involved in hydrogen bond, we confirm that the hydrogen bond O1‐H2?N3 of 2PYP should be strengthened in the S₁ state. Analyses about frontier molecular orbitals prove that charge redistribution of 2PYP facilitates excited state intramolecular proton transfer process. Via constructing potential energy curves and searching transition state structure, we clarify the excited state intramolecular proton transfer mechanism of 2PYP in detail, which may make contributions for the applications of such kinds of system in future.     

Elaborating the excited state behavior of 2‐(4′‐N,N‐dimethylaminophenyl)‐imidazo[4,5‐c]pyridine coupling with methanol solvent

We present a theoretical investigation about the excited state dynamical mechanism of 2‐(4′‐N,N‐dimethylaminophenyl)‐imidazo[4,5‐c]pyridine (DMAPIP‐c). Within the framework of density functional theory and time‐dependent density functional theory methods, we reasonably repeat the experimental electronic spectra, which further confirm the theoretical level used in this work is feasible. Given the best complex model, 3 methanol (MeOH) solvent molecules should be connected with DMAPIP‐c forming DMAPIP‐c‐MeOH complex in both ground state and excited state. Exploring the changes about bond lengths and bond angles involved in hydrogen bond wires, we find the O7‐H8···N9 one should be largely strengthened in the S₁ state, which plays an important role in facilitating the excited state intermolecular proton transfer (ESIPT) process. In addition, the analyses about infrared vibrational spectra also confirm this conclusion. The redistribution about charges distinguished via frontier molecular orbitals based on the photoexcitation, we do find tendency of ESIPT reaction due to the most charges located around N9 atom in the lowest unoccupied molecular orbital. Based on constructing the potential energy curves of both S₀ and S₁ states, we not only confirm that the ESIPT process should firstly occur along with hydrogen bond wire O7‐H8···N9, but also find a low potential energy barrier 8.898 kcal/mol supports the ESIPT reaction in the S₁ state forming DMAPIP‐c‐MeOH‐PT configuration. Subsequently, DMAPIP‐c‐MeOH‐PT could twist its dimethylamino moiety with a lower barrier 3.475 kcal/mol forming DMAPIP‐c‐MeOH‐PT‐TICT structure. Our work not only successfully explains previous experimental work but also paves the way for the further applications about DMAPIP‐c sensor in future.     

A theoretical study on the ESPT mechanism for a novel Bis‐HPBT fluorophore

In this work, based on the density functional theory and time‐dependent density functional theory methods, the properties of the 2 intramolecular hydrogen bonds (O1‐H2···N3 and O4‐H5···N6) of a new photochemical sensor 4‐(3‐(benzo[d]thiazol‐2‐yl)‐5‐tert‐butyl‐4‐hydroxybenzyl)‐2‐(benzo[d]thiazol‐2‐yl)‐6‐tert‐butyl phenol (Bis‐HPBT) have been investigated in detail. The calculated dominating bond lengths and bond angles about these 2 hydrogen bonds (O1‐H2···N3 and O4‐H5···N6) demonstrate that the intramolecular hydrogen bonds should be strengthened in the S₁ state. In addition, the variations of hydrogen bonds of Bis‐HPBT have been also testified based on infrared vibrational spectra. Our theoretical results reproduced absorption and emission spectra of the experiment, which verifies that the theoretical level we used is reasonable and effective in this work. Further, hydrogen bonding strengthening manifests the tendency of excited state intramolecular proton transfer (ESIPT) process. Frontier molecular orbitals depict the nature of electronically excited state and support the ESIPT reaction. According to the calculated results of potential energy curves along stepwise and synergetic O1‐H2 and O4‐H5 coordinates, the potential energy barrier of approximately 1.399 kcal/mol is discovered in the S₁ state, which supports the single ESIPT process along with 1 hydrogen bond of Bis‐HPBT. In other words, the proton transfer reaction can be facilitated based on the electronic excitation effectively. In turn, through the process of radiative transition, the proton‐transfer Bis‐HPBT‐SPT form regresses to the ground state with the fluorescence of 539 nm.     

Theoretical insight into the excited‐state proton transfer process: Role of the substituent ‐CN on HBT system

In this work, using density functional theory and time‐dependent density functional theory methods, we theoretically studied the excited‐state behaviors of 3 novel 2‐(2‐hydroxyphenyl)benzothiazole (HBT) derivatives (HBT‐H‐H, HBT‐CN‐H, and HBT‐CN‐CN). Analyses about primary chemical structures such as bond lengths and bond angles, we found that all the intramolecular hydrogen bonds in these 3 structures should be strengthened in the S₁ state upon the photoexcitation. Exploring the infrared vibrational spectra at the hydrogen bonds groups, we confirmed that nonsubstitutional HBT‐H‐H structure might play more important roles in the excited‐state intramolecular proton transfer (ESIPT) reaction than HBT‐CN‐H and HBT‐CN‐CN. Further, investigating vertical excitation process, it can be revealed that charge redistribution involved in hydrogen bonding moieties could facilitate the ESIPT reaction. Based on constructing potential energy curves of both S₀ and S₁ states, we confirmed that the substituents on HBT systems can reasonably regulate and control the ESIPT processes because of the different potential energy barriers. We deem that this present work not only elaborates the different excited‐state behaviors of HBT‐H‐H, HBT‐CN‐H, and HBT‐CN‐CN but also may play important roles in designing and developing new materials and applications involved in HBT systems in future.     

Theoretical insight into the excited‐state behavior of a novel Compound 1: A TDDFT investigation

In the present work, using density functional theory and time‐dependent density functional theory methods, we investigated and presented the excited‐state intramolecular proton transfer (ESIPT) mechanisms of a novel Compound 1 theoretically. Analyses of electrostatic potential surfaces and reduced density gradient (RDG) versus sign(λ₂)ρ, we confirm the existence of intramolecular hydrogen bond O1‐H2···N3 for Compound 1 in the S₀ state. Comparing the primary structural variations of Compound 1 involved in the intramolecular hydrogen bond, we find that O1‐H2···N3 should be strengthened in the S₁ state, which may facilitate the ESIPT process. Concomitantly, infrared (IR) vibrational spectra analyses further verify the stability of hydrogen bond. In addition, the role of charge transfer interaction has been addressed under the frontier molecular orbitals, which depicts the nature of electronical excited state and supports the ESIPT reaction. The theoretically scanned and optimized potential energy curves according to variational O1‐H2 coordinate demonstrate that the proton transfer process should occur spontaneously in the S₁ state. It further explains why the emission peak of Compound 1‐enol was not reported in previous experiment. This work not only presents the ESIPT mechanism of Compound 1 but also promotes the understanding of this kind of molecules for further applications in future.     

TDDFT study on the excited‐state hydrogen bonding of N‐(2‐hydroxyethyl)‐1,8‐naphthalimide and N‐(3‐hydroxyethyl)‐1,8‐naphthalimide in methanol solution

The time‐dependent density functional theory method was performed to investigate the excited‐state hydrogen‐bonding dynamics of N‐(2‐hydroxyethyl)‐1,8‐naphthalimide (2a) and N‐(3‐hydroxyethyl)‐1,8‐naphthalimide (3a) in methanol (meoh) solution. The ground and excited‐state geometry optimizations, electronic excitation energies, and corresponding oscillation strengths of the low‐lying electronically excited states for the complexes 2a + 2meoh and 3a + 2meoh as well as their monomers 2a and 3a were calculated by density functional theory and time‐dependent density functional theory methods, respectively. We demonstrated that the three intermolecular hydrogen bonds of 2a + 2meoh and 3a + 2meoh are strengthened after excitation to the S₁ state, and thus induce electronic spectral redshift. Moreover, the electronic excitation energies of the hydrogen‐bonded complexes in S₁ state are correspondingly decreased compared with those of their corresponding monomer 2a and 3a. In addition, the intramolecular charge transfer of the S₁ state for complexes 2a + 2meoh and 3a + 2meoh were theoretically investigated by analysis of molecular orbital. Copyright © 2014 John Wiley & Sons, Ltd.     

Exploring excited‐state proton transfer mechanism for 9,10‐dihydroxybenzo[h]quinolone

In this work, we mainly focus on the excited‐state intramolecular proton transfer mechanism of a new molecule 9,10‐dihydroxybenzo[h]quinoline (9‐10‐HBQ). Within the framework of density functional theory and time‐dependent density functional theory methods, we have theoretically investigated its excited‐state dynamical process and our theoretical results successfully reappeared previous experimental electronic spectra. The ultrafast excited‐state intramolecular proton transfer process occurs in the first excited state (S₁ state) forming 9‐10‐HBQ‐PT1 structure without potential energy barrier along with hydrogen bond (O₃–H₄···N₅). Then the second proton may transfer via another intramolecular hydrogen bonded wire (O₁–H₂···N₃) with a moderate potential energy barrier (about 7.69 kcal/mol) in the S₁ state forming 9‐10‐HBQ‐PT2 configuration. After completing excited‐state dynamical process, the molecule on the first excited electronic state would come back to the ground state. We not only clarify the excited‐state dynamical process for 9‐10‐HBQ but also put forward new predictions and successfully explain previous experimental results.     

A theoretical investigation about sensing mechanism of fluoride anion for (E)‐2‐(2‐(dimethylamino)ethyl)‐6‐(4‐hydroxystyryl)‐1H‐benzo[de]‐isoquinoline‐1,3 (2H)‐dione

In the present work, we theoretical study the sensing mechanism of a new fluoride chemosensor (E)‐2‐(2‐(dimethylamino)ethyl)‐6‐(4‐hydroxystyryl)‐1H‐benzo[de]‐isoquinoline‐1,3(2H)‐dione (the abbreviation is NIM ). Based on density functional theory and time‐dependent density functional theory methods, the fluoride anion response mechanism has been confirmed via constructing potential energy curve. The exothermal deprotonation process along with the intermolecular hydrogen bond O–H···F reveals the uniqueness of detecting F^?. After capturing hydrogen proton forming NIM‐A anion configuration, a new absorption peak around 655 nm appears in dimethyl sulfoxide solvent. In addition, the emission of NIM can be quenched when adding F^? has been also confirmed. Due to the twisted intramolecular charge transfer character NIM‐A‐S ₁ form, we further verify the experimental phenomenon. The theoretical electronic spectra (vertical excitation energies and fluorescence peak) reproduced previous experimental results (ACS Appl. Mater. Interfaces 2014, 6, 7996), which not only reveals the rationality of our theoretical level used in this work but also confirms the correctness of geometrical attribution. In view of the excitation process, the strong intramolecular charge transfer process of S₀ → S₁ transition explain the redshift of absorption peak for NIM with the addition of fluoride anion. This work presents a straightforward sensing mechanism (deprotonation process) of fluoride anion for the novel NIM chemosensor.     

10-羟基苯并喹啉激发态分子内质子转移取代基效应的理论研究

运用密度泛函(DFT)和含时密度泛函(TDDFT)计算方法研究了10-羟基苯并喹啉(HBQ)及其衍生物分子内质子转移过程,探究了取代基效应对质子转移过程的影响,研究发现,HBQ及其衍生物可以形成分子内氢键,且激发态时氢键增强.基态时各分子以醇式构型稳定存在,激发态时酮式构型为优势构象.各化合物的最大吸收峰和发射峰主要是电子从HOMO到LUMO之间的跃迁引起的.基态分子内质子转移(醇式→酮式)需要跃过较高的能垒因而难以发生,而激发态时只需跃过较低能垒就很容易发生分子内质子转移,吸电子基的引入可以使该过程的能垒降低,因此吸电子基有利于激发态质子转移.取代基效应影响化合物的光谱性质.     

Theoretical design on a new double functional device of 2,2′‐bipyridine‐embedded N‐(9‐pyrenyl methyl)aza‐15‐crown‐5

Theoretical design on a new molecular switch and fluorescent chemosensor double functional device of aza‐crown ether (2,2′‐dipyridine‐embedded N‐(9‐anthraceneyl(pyrenyl)methyl)aza‐15‐crown‐5) was explored. The interactions between ligands and a series of alkaline earth metal cations (Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺) were investigated. The fully optimized geometry structures of the free ligands ( L _1, L ₂) and their metal cation complexes ( L ₁/M²⁺, L ₂/M²⁺) were calculated with the B3LYP/6‐31G(d) method. The natural bond orbital analysis, which is based on optimized geometric structures, was used to explore the interaction of L ₁/M²⁺, L ₂/M²⁺ molecules. The absorption spectra of L _1, L ₂, L ₁/M²⁺, and L ₂/M²⁺, and their excited states were studied by time‐dependent density functional theory. A new type molecular device L ₂(2,2′‐dipyridine‐embedded N‐(9‐pyrenyl methyl)aza‐15‐crown‐5) is designed, which not only has the selectivity for Sr²⁺, and construct allosteric switch, but also has fluorescent sensor performance.     

Synthesis and vibrational analysis of N‐(2′‐Furyl)‐Imidazole

The N‐(2′‐furyl)‐imidazole ( 1 ) has been prepared and characterized using infrared, Raman and multidimensional nuclear magnetic resonance spectroscopies. Theoretical calculations have been carried out by employing the Density Functional Theory (DFT) method, in order to optimize the geometry of their two conformers in the gas phase and to support the assignments of the vibrational bands of 1 to their normal modes. For a complete assignment of the compound, DFT calculations were combined with Scaled Quamtum Mecanic Force Field (SQMFF) methodology in order to fit the theoretical wavenumber values to the experimental one. Furthermore, Natural Bond Orbital (NBO) and topological properties by Atoms In Molecules (AIM) calculations were performed to analyze the nature and magnitude of the intramolecular interactions. The result reveals that two conformers are expected in liquid phase. Copyright © 2009 John Wiley & Sons, Ltd.     

FT‐IR,FT‐Raman,SERS spectra and computational calculations of 4‐ethyl‐N‐(2′‐hydroxy‐5′‐nitrophenyl)benzamide

Fourier transform infrared (FT‐IR) and FT‐Raman spectra of 4‐ethyl‐N‐(2′‐hydroxy‐5′‐nitrophenyl)benzamide were recorded and analyzed. A surface‐enhanced Raman scattering (SERS) spectrum was recorded in silver colloid. The vibrational wavenumbers and corresponding vibrational assignments were examined theoretically using the Gaussian03 set of quantum chemistry codes. The red shift of the NH stretching wavenumber in the infrared spectrum from the computational wavenumber indicates the weakening of the NH bond resulting in proton transfer to the neighboring oxygen atom. The simultaneous IR and Raman activation of the CO stretching mode gives the charge transfer interaction through a π‐conjugated path. The presence of methyl modes in the SERS spectrum indicates the nearness of the methyl group to the metal surface, which affects the orientation and metal molecule interaction. The first hyperpolarizability and predicted infrared intensities are reported. The calculated first hyperpolarizability is comparable with the reported values of similar derivatives and is an attractive subject for future studies of nonlinear optics. Optimized geometrical parameters of the title compound are in agreement with reported structures. Copyright © 2009 John Wiley & Sons, Ltd.     

The effect of benzo‐annelation on intermolecular hydrogen bond and proton transfer of 2‐methyl‐3‐hydroxy‐4(1H)‐quinolone in methanol: A TD‐DFT study

Excited‐state intermolecular or intramolecular proton transfer (ESIPT) reaction has important potential applications in biological probes. In this paper, the effect of benzo‐annelation on intermolecular hydrogen bond and proton transfer reaction of the 2‐methyl‐3‐hydroxy‐4(1H)‐quinolone (MQ) dye in methanol solvent is investigated by the density functional theory and time‐dependent density functional theory approaches. Both the primary structure parameters and infrared vibrational spectra analysis of MQ and its benzo‐analogue 2‐methyl‐3‐hydroxy‐4(1H)‐benzo‐quinolone (MBQ) show that the intermolecular hydrogen bond O₁―H₂?O₃ significantly strengthens in the excited state, whereas another intermolecular hydrogen bond O₃―H₄?O₅ weakens slightly. Simulated electron absorption and fluorescence spectra are agreement with the experimental data. The noncovalent interaction analysis displays that the intermolecular hydrogen bonds of MQ are obviously stronger than that of MBQ. Additionally, the energy profile analysis via the proton transfer reaction pathway illustrates that the ESIPT reaction of MBQ is relatively harder than that of MQ. Therefore, the effect of benzo‐annelation of the MQ dye weakens the intermolecular hydrogen bond and relatively inhibits the proton transfer reaction.     

Theoretical investigation on the excited state intramolecular proton coupled charge transfer phenomenon for a novel fluorophore

We theoretically investigate the excited state behaviors of the novel fluorophore tetraphenylethene‐2‐(2′‐hydroxyphenyl)benzothiazole (TPE‐HBT), which was designed based on the intersection of TPE and HBT, using density functional theory and time‐dependent density functional theory methods. Compared with previous experimental results about fluorescence peaks, our calculated results are in good agreement with experimental data, which further confirms that the theoretical level we used is reasonable. Furthermore, our results confirm that the excited state intramolecular proton transfer (ESIPT) process happens upon photoexcitation, which is distinctly monitored by the infrared spectra and the potential energy curves. In addition, the calculation of highest occupied molecular orbital and lowest unoccupied molecular orbital reveals that the electron density change of proton acceptor because of the intramolecular charge transfer (ICT) process in the S₁ state induces the ESIPT. Moreover, the transition density matrix is worked out to facilitate deeper insight into the ESIPT coupled ICT process. It is hoped that the present work not only elaborates the ESIPT coupled ICT phenomenon and corresponding mechanisms for the TPE‐HBT but also may be helpful to design and develop new materials and applications involved in TPE‐HBT systems in future.     

Periodic density functional theory study of the high‐pressure behavior of crystalline 7,2′‐anhydro‐β‐d‐arabinosylorotidine

In this work, a detailed study of the structural, electronic, and absorption properties of crystalline 7,2′‐anhydro‐β‐d ‐arabinosylorotidine (Cyclo ara‐O) in the pressure range of 0–350 GPa is performed by density functional theory calculations. The detail analysis of the crystal with increasing pressure shows that complex transformations occur in Cyclo ara‐O under compression. In addition, the b‐direction is much stiffer than the a‐ and c‐axis at 0–330 GPa, suggesting that the Cyclo ara‐O crystal is anisotropic in the certain pressure region. In the pressure range of 110–290 GPa, repeated formations and disconnections of covalent bonds in O7–O6* and C3–C6* occur several times, resulting in a new six‐atom ring that forms at 220, 270, and 290 GPa, while a five‐atom ring and seven‐atom ring form between two adjacent molecules at 300 and 340 GPa, respectively. Then, the analysis of the band gap and DOS (PDOS) of Cyclo ara‐O indicates that its electronic character has changed at 300 GPa into an excellent insulator, but the electron transition is much easier at 350 GPa. Moreover, the relatively high optical activity with the pressure increases of Cyclo ara‐O is seen from the absorption spectra, and two obvious structural transformations are also observed at 180 and 230 GPa, respectively. Copyright © 2016 John Wiley & Sons, Ltd.     

Density functional theory calculation and vibrational spectral analysis of 4‐hydroxy‐3‐(3‐oxo‐1‐phenylbutyl)‐2H‐1‐benzopyran‐2‐one

The Fourier transform infrared (4000–400 cm⁻¹) and Fourier transform Raman (3500–500 cm⁻¹) spectra of 4‐hydroxy‐3‐(3‐oxo‐1‐phenylbutyl)‐2H‐1‐benzopyran‐2‐one (Warfarin) have been measured and calculated. The structure optimization has been made using density functional theory (DFT) calculations. Complete vibrational assignments of the observed spectra have been compared with theoretical wavenumbers. The wavenumber increasing in the methyl group shows the electronic hyperconjugation effect. The natural bond orbital (NBO) analysis reveals the hyperconjugation interaction and the intramolecular hydrogen bonding. The first‐order hyperpolarizability has been calculated. Copyright © 2009 John Wiley & Sons, Ltd.     

FT‐IR,FT‐Raman,and computational calculations of 4‐chloro‐2‐(3‐chlorophenyl carbamoyl)phenyl acetate

FT‐IR and FT‐Raman spectra of 4‐chloro‐2‐(3‐chlorophenylcarbamoyl) phenyl acetate were studied. Vibrational wavenumbers and corresponding vibrational assignments were examined theoretically using the Gaussian03 set of quantum chemistry codes and the normal modes are assigned by potential energy distribution (PED) calculations. Simultaneous IR and Raman activation of the CO stretching mode shows the charge transfer interaction through a π‐conjugated path. Optimized geometrical parameters of the title compound are in agreement with the reported values. Analysis of the phenyl ring modes shows that C C stretching mode is equally active as strong bands in both IR and Raman, which can be interpreted as the evidence of intramolecular charge transfer via conjugated ring path and is responsible for hyperpolarizability enhancement leading to nonlinear optical activity. The red‐shift of the NH‐stretching wavenumber in the infrared spectrum from the computed wavenumber indicates the weakening of the NH bond resulting in proton transfer to the neighboring oxygen atom. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis and structural study of 2′‐ and 2′,6′‐positioned methyl‐ and nitro‐substituted 3‐(arylhydrazono)pentane‐2,4‐diones

A systematic series of ortho‐methyl‐ and nitro‐substituted arylhydrazones 2–6 formed by Japp–Klingemann reaction between pentane‐2,4‐dione and the respective aryldiazonium salts have been synthesized and studied by X‐ray crystal structure analysis, with added quantum chemical calculations. The optimized molecular geometries based on DFT calculations, enabling determination of relevant rotational barriers, and the calculated bond and ring critical points, using the method of ‘atoms in molecules’, were found to correspond with the experimental data, involving specific molecular conformations and hydrogen‐bonded ring structure dependent on the ortho‐substitution, thus making possible reliable structural prediction of this compound class. Copyright © 2007 John Wiley & Sons, Ltd.