Solid-state and solution pH-dependent NEXAFS studies allow direct observation of the electronic state of para-aminobenzoic acid (PABA) as a function of its chemical environment, revealing the chemical state and bonding of the chemical species. Variations in the ionization potential (IP) and 1s→π* resonances unequivocally identify the chemical species (neutral, cationic, or anionic) present and the varying local environment. Shifts in σ* shape resonances relative to the IP in the NEXAFS spectra vary with C-N bond length, and the important effect of minor alterations in bond length is confirmed with nitrogen FEFF calculations, leading to the possibility of bond length determination in solution

Aziz, EF

Gainar, A

Golnak, R

Schroeder, SLM

Stevens, JS

Suljoti, E

Xiao, J

Journal of Physics : Conference Series

Stevens, J.S.

Gainar, A.

Suljoti, E.

Xiao, J.

Golnak, R.

Aziz, E.F.

Schroeder, S.L.M.

HZB Repository

This content has been downloaded from IOPscience. Please scroll down to see the full text.Download details:IP Address: 129.11.23.82This content was downloaded on 06/06/2016 at 14:27Please note that terms and conditions apply.NEXAFS Chemical State and Bond Lengths of p-Aminobenzoic Acid in Solution and SolidStateView the table of contents for this issue, or go to the journal homepage for more2016 J. Phys.: Conf. Ser. 712 012136(http://iopscience.iop.org/1742-6596/712/1/012136)Home Search Collections Journals About Contact us My IOPscience      NEXAFS Chemical State and Bond Lengths of p-Aminobenzoic Acid in Solution and Solid State J S Stevens1, A Gainar1, E Suljoti2, J Xiao2, R Golnak2, E F Aziz2,3, S L M Schroeder4,5 1School of Chemical Engineering and Analytical Science, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK 2Joint Ultrafast Dynamics Lab in Solutions and at Interfaces (JULiq), Helmholtz-Zentrum Berlin für Materialien und Energie, Albert Einstein Strasse 15, 12489 Berlin, Germany 3Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany 4School of Chemical and Process Engineering, University of Leeds, Leeds, LS2 9JT, UK 5DIAMOND Light Source Ltd., Chilton, Didcot, OX11 0QX, UK  E-mail: joanna.stevens@manchester.ac.uk; s.l.m..schroeder@leeds.ac.uk Abstract. Solid-state and solution pH-dependent NEXAFS studies allow direct observation of the electronic state of para-aminobenzoic acid (PABA) as a function of its chemical environment, revealing the chemical state and bonding of the chemical species. Variations in the ionization potential (IP) and 1s→π* resonances unequivocally identify the chemical species (neutral, cationic, or anionic) present and the varying local environment. Shifts in σ* shape resonances relative to the IP in the NEXAFS spectra vary with C−N bond length, and the important effect of minor alterations in bond length is confirmed with nitrogen FEFF calculations, leading to the possibility of bond length determination in solution. 1.  Introduction Through X-ray absorption and emission spectroscopies, the chemical, electronic and structural properties of organic species can be investigated. The nature of solution species and their interactions can play an important role in the activity of biological systems [1] and in the structural outcome and therefore properties of crystallization products [2]. Near-edge X-ray absorption fine structure (NEXAFS) measurements allow direct observation of the electronic state of para-aminobenzoic acid (PABA) as a function of its chemical environment [3]. While the IP and π* resonances are characteristic of the species and related to their chemical and electronic nature in the solution and solid state, the dominating influence of bond length on the σ* shape resonance proposed from experimental data is here investigated with FEFF calculations, exploring the ability and sensitivity of measuring bond lengths.      16th International Conference on X-ray Absorption Fine Structure (XAFS16) IOP PublishingJournal of Physics: Conference Series 712 (2016) 012136 doi:10.1088/1742-6596/712/1/012136Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing Ltd 1      2.  Methods 2.1.  N K-edge Near Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy Solid-state partial electron yield (PEY) NEXAFS measurements were performed on the U7a beamline of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory, NY with the sample at 54.7°, a 600 l/mm monochromator grating, and an entrance grid bias of -150 V [3]. Spectra were normalized by the simultaneously recorded total electron yield signal from a gold-coated, 90% transmission grid in the incident beam (I0), and the monochromator energy scale was calibrated using the 400.6 eV 1st π* of a titanium nitride grid. Solution-state partial fluorescence yield (PFY) spectra were recorded using the liquid microjet of the LiXEdrom endstation at the U41-PGM beamline of the BESSY II synchrotron at Helmholtz Zentrum Berlin (HZB) using an 18 µm diameter glass nozzle with 0.6 mL min-1 flow rate [3]. Beamline energy calibration was performed with N2 gas (total electron yield X-ray absorption spectroscopy 1s→2p π transition). Peak fitting and normalization were performed using the Athena software [4,5], with arctan steps for the ionization potentials (IPs, edge steps) and Gaussian functions for the peaks. 2.2.  FEFF calculations FEFF8.2 [6] was used to simulate the σ* shape resonance in the N K-edge NEXAFS for a geometry optimized monomer of non-ionic PABA. The SCF (7.0), XANES (4.0), and FMS (10.0) cards were used, and the calculated data were rigid shifted by +2.3 eV. To identify the influence of the C–N bond length, the C–N bond length of α-PABA was increased by 0.002-0.1 Å relative to the equilibrium value for the non-ionic monomer.  3.  Results and Discussion Nitrogen K-edge NEXAFS measurements reveal variations for the PABA species in both the solid state and in solution (Figure 1). Characteristic spectra are obtained for the different chemical species (non-ionic, cationic, anionic) present in solution with variation in pH, with a distinctive electronic signature for the cationic form following the change in nitrogen protonation state (NH2 to NH3+). Shifts in the ionization potential (IP, edge step) and 1s→π* resonances unequivocally identify the chemical and electronic nature of the species present [3,7]. Further slight shifts and intensity variations are observed for the impact of differing local environments and interactions for the three non-ionic species investigated (α- and β-crystalline forms and the solutions species in methanol).  The energy of the σ* resonance associated with the C−N bond relative to the IP also varies across the series (term value δC-N, Figure 1). The decrease in δC-N appears to correlate well with successively longer C−N bond lengths reported by X-ray diffraction (XRD) for the solid-state (Table 1, y = -39.664x + 61.133) [3]. This relationship can then be applied to the NEXAFS data, using it to provide an indicator of bond lengths for PABA species in solution.  Table 1. N K-edge NEXAFS features and bond lengths [3].  δC-N  / eV ∆δC-N  / eV  XRD  C−N / Å XRD lengthening  C−N / Å NEXAFS C−N / Åb Non-ionic α-PABA 6.31 0 1.379(2) 0 1.382 Non-ionic β-PABA 5.49 -0.82 1.408(3) 0.029 1.403 Anionic pH 11 5.40 -0.91 1.410(2)a 0.031 1.405 Non-ionic Methanol 5.19 -1.12 / / 1.411 Cationic pH 1 3.16 -3.15 / / 1.462 Cationic HCl salt 3.05 -3.26 1.4626(5) 0.0836 1.464 a C−N bond length from the hydrated Na salt shown for comparison b y = -39.664x + 61.133 for linear relationship between δC-N and XRD C−N/Å [3]. 16th International Conference on X-ray Absorption Fine Structure (XAFS16) IOP PublishingJournal of Physics: Conference Series 712 (2016) 012136 doi:10.1088/1742-6596/712/1/0121362       Figure 1. Nitrogen K-edge NEXAFS for anionic, non-ionic, and cationic PABA species in the solid and solution state, with the IPs marked by grey lines and the magnitude of the term value δC-N highlighted by grey boxes (adapted under CC BY license [3,7]).  To further investigate the impact of the C−N bond of PABA on the nitrogen NEXAFS, FEFF calculations were performed. Using an optimized non-ionic monomer as input, only the C−N length was varied, keeping all other parameters constant. This enables the effect of the C−N bond length to be isolated from other changes occurring between the species.  The C−N length was initially extended by 0.1 Ångstroms (Å), where a clear alteration and decrease in σ* energy of almost 2.5 eV is observed relative to the IP (∆δC-N, Figure 2). The changes in C−N bond length in practice are smaller than 0.1 Å, ranging from 0.002 to 0.08 Å for the different PABA species (Table 1). Smaller increments were therefore tested, with lengthening by 0.03 Å (to mimic that occurring on moving from non-ionic α-PABA to β- or anionic PABA) resulting in a decrease in 16th International Conference on X-ray Absorption Fine Structure (XAFS16) IOP PublishingJournal of Physics: Conference Series 712 (2016) 012136 doi:10.1088/1742-6596/712/1/0121363      energy by around 0.8 eV (Figure 2). This compares favourably with experimental NEXAFS, where a 0.82-0.91 eV change occurs (for cationic species, underestimation of FEFF ∆δC-N values may be related to the alteration in electronic structure and geometry accompanying the positive charge on N, bent NH2 → tetrahedral NH3+). The extreme sensitivity is further demonstrated for the case with just 0.005 Å longer C−N (Figure 2), and even the small shift occurring with 0.002 Å lengthening is observable, showing that even relatively subtle variations have a noticeable influence. The results suggest that bond length variations should be quantifiable to a precision of approximately 0.01 Å.  -2.5-2-1.5-1-0.500 0.02 0.04 0.06 0.08 0.1∆δ C−N/ eV C−N bond length elongation / Å1.522.533.544.55-5 -4 -3 -2 -1 0 1 2 3 4 5µ(E) / a.u.∆δ C−N / eVPABA0.002 Å longer C-N0.005 Å longer C-N0.01 Å longer C-N0.03 Å longer C-N0.08 Å longer C-N0.1 Å longer C-N  Figure 2. FEFF calculations for the nitrogen K-edge NEXAFS of PABA, demonstrating the effect of lengthening the C−N bond on the σ* energy relative to IP (δ*C-N). 4.  Conclusions Nitrogen NEXAFS acts as a chemical and structural indicator for PABA, identifying the chemical species present in both the solid-state and in solution as well as providing a measure of C−N bond length. The ability to detect even minor variations in structure is verified by the FEFF calculations through incremental variation of bond length, with changes from as little as 0.002 Å alteration visible. NEXAFS can then potentially provide bond lengths for solution (or amorphous) species, such as demonstrated with PABA, and the combination of experiment and calculations is particularly strong. Acknowledgements Use of the NSLS at BNL was supported by U.S. Department of Energy Contract No. DE-AC02-98CH10886, and BESSY II at HZB by European Community’s Seventh Framework Programme, grant no. 312284. JSS, AG and SLMS acknowledge support by EPSRC Critical Mass Grant EP/I013563/1. References [1] Chatterjea M N and Shinde R. 2012 Textbook of Medical Biochemistry (New Delhi: Jaypee Brothers Medical Publishers (P) Ltd.) [2] Davey R. J., Schroeder S L M, and ter Horst J H 2013 Angew. Chem., Int. Ed. 52 2166-2179 [3] Stevens J S, Gainar A, Suljoti E, Xiao J, Golnak R, Aziz E F and Schroeder S L M 2015 Chem. - Eur. J. 21 7256-7263 [4] Ravel B and Newville M 2005 J. Synchrotron Radiat. 12 537-541 [5] Newville M 2001 J. Synchrotron Radiat. 8 322-324 [6] Ankudinov A L, Ravel B, Rehr J J and Conradson S D, 1998 Phys. Rev. B: Condens. Matter Mater. Phys. 58 7565-7576 [7] Stevens J S, Seabourne C R, Jaye C, Fischer D A, Scott A J and Schroeder S L M 2014 J. Phys. Chem. B 118 12121-12128 16th International Conference on X-ray Absorption Fine Structure (XAFS16) IOP PublishingJournal of Physics: Conference Series 712 (2016) 012136 doi:10.1088/1742-6596/712/1/0121364

Undetermined

NEXAFS Chemical State and Bond Lengths of p Aminobenzoic Acid in Solution and Solid State

White Rose Research Online

NEXAFS Chemical State and Bond Lengths of p-Aminobenzoic Acid in Solution and Solid State

J S Stevens

A Gainar

E Suljoti

J Xiao

R Golnak

E F Aziz

S L M Schroeder

Crossref

Journal of Physics Conference Series

Solid-state and solution pH-dependent NEXAFS studies allow direct observation
of the electronic state of para-aminobenzoic acid (PABA) as a function of its
chemical environment, revealing the chemical state and bonding of the chemical
species. Variations in the ionization potential (IP) and 1s→π* resonances
unequivocally identify the chemical species (neutral, cationic, or anionic)
present and the varying local environment. Shifts in σ* shape resonances
relative to the IP in the NEXAFS spectra vary with C-N bond length, and the
important effect of minor alterations in bond length is confirmed with
nitrogen FEFF calculations, leading to the possibility of bond length
determination in solution

Stevens, J. S.

Aziz, Emad F.

Schroeder, S. L. M.

Institutional Repository of the Freie Universität Berlin

This content has been downloaded from IOPscience. Please scroll down to see the full text.Download details:IP Address: 160.45.66.177This content was downloaded on 27/02/2017 at 10:57Please note that terms and conditions apply.NEXAFS Chemical State and Bond Lengths of p-Aminobenzoic Acid in Solution and SolidStateView the table of contents for this issue, or go to the journal homepage for more2016 J. Phys.: Conf. Ser. 712 012136(http://iopscience.iop.org/1742-6596/712/1/012136)Home Search Collections Journals About Contact us My IOPscienceYou may also be interested in:The Structure of p-Aminobenzoic Acid in Water: Studies Combining UV-Vis, NEXAFS and RIXSSpectroscopiesA Gainar, J S Stevens, E Suljoti et al.Calculation of Bond Lengths in InGaAsP Quaternary Alloy SemiconductorMasaya Ichimura and Akio Sasaki      NEXAFS Chemical State and Bond Lengths of p-Aminobenzoic Acid in Solution and Solid State J S Stevens1, A Gainar1, E Suljoti2, J Xiao2, R Golnak2, E F Aziz2,3, S L M Schroeder4,5 1School of Chemical Engineering and Analytical Science, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK 2Joint Ultrafast Dynamics Lab in Solutions and at Interfaces (JULiq), Helmholtz-Zentrum Berlin für Materialien und Energie, Albert Einstein Strasse 15, 12489 Berlin, Germany 3Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany 4School of Chemical and Process Engineering, University of Leeds, Leeds, LS2 9JT, UK 5DIAMOND Light Source Ltd., Chilton, Didcot, OX11 0QX, UK  E-mail: joanna.stevens@manchester.ac.uk; s.l.m..schroeder@leeds.ac.uk Abstract. Solid-state and solution pH-dependent NEXAFS studies allow direct observation of the electronic state of para-aminobenzoic acid (PABA) as a function of its chemical environment, revealing the chemical state and bonding of the chemical species. Variations in the ionization potential (IP) and 1s→p* resonances unequivocally identify the chemical species (neutral, cationic, or anionic) present and the varying local environment. Shifts in s* shape resonances relative to the IP in the NEXAFS spectra vary with C-N bond length, and the important effect of minor alterations in bond length is confirmed with nitrogen FEFF calculations, leading to the possibility of bond length determination in solution. 1.  Introduction Through X-ray absorption and emission spectroscopies, the chemical, electronic and structural properties of organic species can be investigated. The nature of solution species and their interactions can play an important role in the activity of biological systems [1] and in the structural outcome and therefore properties of crystallization products [2]. Near-edge X-ray absorption fine structure (NEXAFS) measurements allow direct observation of the electronic state of para-aminobenzoic acid (PABA) as a function of its chemical environment [3]. While the IP and p* resonances are characteristic of the species and related to their chemical and electronic nature in the solution and solid state, the dominating influence of bond length on the s* shape resonance proposed from experimental data is here investigated with FEFF calculations, exploring the ability and sensitivity of measuring bond lengths.      16th International Conference on X-ray Absorption Fine Structure (XAFS16) IOP PublishingJournal of Physics: Conference Series 712 (2016) 012136 doi:10.1088/1742-6596/712/1/012136Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing Ltd 1      2.  Methods 2.1.  N K-edge Near Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy Solid-state partial electron yield (PEY) NEXAFS measurements were performed on the U7a beamline of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory, NY with the sample at 54.7°, a 600 l/mm monochromator grating, and an entrance grid bias of -150 V [3]. Spectra were normalized by the simultaneously recorded total electron yield signal from a gold-coated, 90% transmission grid in the incident beam (I0), and the monochromator energy scale was calibrated using the 400.6 eV 1st p* of a titanium nitride grid. Solution-state partial fluorescence yield (PFY) spectra were recorded using the liquid microjet of the LiXEdrom endstation at the U41-PGM beamline of the BESSY II synchrotron at Helmholtz Zentrum Berlin (HZB) using an 18 mm diameter glass nozzle with 0.6 mL min-1 flow rate [3]. Beamline energy calibration was performed with N2 gas (total electron yield X-ray absorption spectroscopy 1s→2p p transition). Peak fitting and normalization were performed using the Athena software [4,5], with arctan steps for the ionization potentials (IPs, edge steps) and Gaussian functions for the peaks. 2.2.  FEFF calculations FEFF8.2 [6] was used to simulate the σ* shape resonance in the N K-edge NEXAFS for a geometry optimized monomer of non-ionic PABA. The SCF (7.0), XANES (4.0), and FMS (10.0) cards were used, and the calculated data were rigid shifted by +2.3 eV. To identify the influence of the C–N bond length, the C–N bond length of a-PABA was increased by 0.002-0.1 Å relative to the equilibrium value for the non-ionic monomer.  3.  Results and Discussion Nitrogen K-edge NEXAFS measurements reveal variations for the PABA species in both the solid state and in solution (Figure 1). Characteristic spectra are obtained for the different chemical species (non-ionic, cationic, anionic) present in solution with variation in pH, with a distinctive electronic signature for the cationic form following the change in nitrogen protonation state (NH2 to NH3+). Shifts in the ionization potential (IP, edge step) and 1s→p* resonances unequivocally identify the chemical and electronic nature of the species present [3,7]. Further slight shifts and intensity variations are observed for the impact of differing local environments and interactions for the three non-ionic species investigated (a- and b-crystalline forms and the solutions species in methanol).  The energy of the s* resonance associated with the C-N bond relative to the IP also varies across the series (term value dC-N, Figure 1). The decrease in dC-N appears to correlate well with successively longer C-N bond lengths reported by X-ray diffraction (XRD) for the solid-state (Table 1, y = -39.664x + 61.133) [3]. This relationship can then be applied to the NEXAFS data, using it to provide an indicator of bond lengths for PABA species in solution.  Table 1. N K-edge NEXAFS features and bond lengths [3].  dC-N  / eV ∆dC-N  / eV  XRD  C-N / Å XRD lengthening  C-N / Å NEXAFS C-N / Åb Non-ionic a-PABA 6.31 0 1.379(2) 0 1.382 Non-ionic b-PABA 5.49 -0.82 1.408(3) 0.029 1.403 Anionic pH 11 5.40 -0.91 1.410(2)a 0.031 1.405 Non-ionic Methanol 5.19 -1.12 / / 1.411 Cationic pH 1 3.16 -3.15 / / 1.462 Cationic HCl salt 3.05 -3.26 1.4626(5) 0.0836 1.464 a C-N bond length from the hydrated Na salt shown for comparison b y = -39.664x + 61.133 for linear relationship between dC-N and XRD C-N/Å [3]. 16th International Conference on X-ray Absorption Fine Structure (XAFS16) IOP PublishingJournal of Physics: Conference Series 712 (2016) 012136 doi:10.1088/1742-6596/712/1/0121362       Figure 1. Nitrogen K-edge NEXAFS for anionic, non-ionic, and cationic PABA species in the solid and solution state, with the IPs marked by grey lines and the magnitude of the term value dC-N highlighted by grey boxes (adapted under CC BY license [3,7]).  To further investigate the impact of the C-N bond of PABA on the nitrogen NEXAFS, FEFF calculations were performed. Using an optimized non-ionic monomer as input, only the C-N length was varied, keeping all other parameters constant. This enables the effect of the C-N bond length to be isolated from other changes occurring between the species.  The C-N length was initially extended by 0.1 Ångstroms (Å), where a clear alteration and decrease in s* energy of almost 2.5 eV is observed relative to the IP (∆dC-N, Figure 2). The changes in C-N bond length in practice are smaller than 0.1 Å, ranging from 0.002 to 0.08 Å for the different PABA species (Table 1). Smaller increments were therefore tested, with lengthening by 0.03 Å (to mimic that occurring on moving from non-ionic a-PABA to b- or anionic PABA) resulting in a decrease in 16th International Conference on X-ray Absorption Fine Structure (XAFS16) IOP PublishingJournal of Physics: Conference Series 712 (2016) 012136 doi:10.1088/1742-6596/712/1/0121363      energy by around 0.8 eV (Figure 2). This compares favourably with experimental NEXAFS, where a 0.82-0.91 eV change occurs (for cationic species, underestimation of FEFF ∆dC-N values may be related to the alteration in electronic structure and geometry accompanying the positive charge on N, bent NH2 → tetrahedral NH3+). The extreme sensitivity is further demonstrated for the case with just 0.005 Å longer C-N (Figure 2), and even the small shift occurring with 0.002 Å lengthening is observable, showing that even relatively subtle variations have a noticeable influence. The results suggest that bond length variations should be quantifiable to a precision of approximately 0.01 Å.  -2.5-2-1.5-1-0.500 0.02 0.04 0.06 0.08 0.1∆d C-N/ eV C-N bond length elongation / Å1.522.533.544.55-5 -4 -3 -2 -1 0 1 2 3 4 5m(E) / a.u.∆d C-N / eVPABA0.002 Å longer C-N0.005 Å longer C-N0.01 Å longer C-N0.03 Å longer C-N0.08 Å longer C-N0.1 Å longer C-N  Figure 2. FEFF calculations for the nitrogen K-edge NEXAFS of PABA, demonstrating the effect of lengthening the C-N bond on the s* energy relative to IP (d*C-N). 4.  Conclusions Nitrogen NEXAFS acts as a chemical and structural indicator for PABA, identifying the chemical species present in both the solid-state and in solution as well as providing a measure of C-N bond length. The ability to detect even minor variations in structure is verified by the FEFF calculations through incremental variation of bond length, with changes from as little as 0.002 Å alteration visible. NEXAFS can then potentially provide bond lengths for solution (or amorphous) species, such as demonstrated with PABA, and the combination of experiment and calculations is particularly strong. Acknowledgements Use of the NSLS at BNL was supported by U.S. Department of Energy Contract No. DE-AC02-98CH10886, and BESSY II at HZB by European Community’s Seventh Framework Programme, grant no. 312284. JSS, AG and SLMS acknowledge support by EPSRC Critical Mass Grant EP/I013563/1. References [1] Chatterjea M N and Shinde R. 2012 Textbook of Medical Biochemistry (New Delhi: Jaypee Brothers Medical Publishers (P) Ltd.) [2] Davey R. J., Schroeder S L M, and ter Horst J H 2013 Angew. Chem., Int. Ed. 52 2166-2179 [3] Stevens J S, Gainar A, Suljoti E, Xiao J, Golnak R, Aziz E F and Schroeder S L M 2015 Chem. - Eur. J. 21 7256-7263 [4] Ravel B and Newville M 2005 J. Synchrotron Radiat. 12 537-541 [5] Newville M 2001 J. Synchrotron Radiat. 8 322-324 [6] Ankudinov A L, Ravel B, Rehr J J and Conradson S D, 1998 Phys. Rev. B: Condens. Matter Mater. Phys. 58 7565-7576 [7] Stevens J S, Seabourne C R, Jaye C, Fischer D A, Scott A J and Schroeder S L M 2014 J. Phys. Chem. B 118 12121-12128 16th International Conference on X-ray Absorption Fine Structure (XAFS16) IOP PublishingJournal of Physics: Conference Series 712 (2016) 012136 doi:10.1088/1742-6596/712/1/0121364

NEXAFS Chemical State and Bond Lengths of p-Aminobenzoic Acid in Solution and
Solid State

Stevens, Joanna

Gainar, Adrian

Suljoti, Edlira

Xiao, Jie

Golnak, Ronny

Aziz, Emad Flear

Schroeder, Sven L. M.

The University of Manchester - Institutional Repository

Nitrogen  K-edge  XPS  and  NEXAFS  of  the  two  polymorphic  forms  of  para-
aminobenzoic acid (PABA) are significantly different reflecting variation in hydrogen bonding. 
Alteration in hydrogen bonding at the amino group leads to a shift to high energy for both the 
XPS N 1s core level and the 3
p
* NEXAFS resonance with 
b
-PABA. Participation of the amine 
group  in  the  aromatic  system  causes  the  1
p
*  resonance  to  be  sensitive  to  the  nature  of  the 
intermolecular  bonding  at  the 
para
-carboxylic  acid  group,  and  a  shift  to  low  energy  for 
a
-
PABA   is   observed   due   to   hydrogen-bonded   carboxylic   acid   dimer   formation.   FEFF 
calculations  also  successfully  reproduce  both  the  energy  and  intensity  variations  observed  for 
the 
s
*  shape  resonance  associated  with  the  C
-
N  bond,  with  the  majority  of  the  decrease  in 
energy observed for 
b
-PABA arising from the longer C
-
N bond

Spiral - Imperial College Digital Repository

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NEXAFS Chemical State and Bond Lengths of p-Aminobenzoic Acid in Solution and Solid State

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The University of Manchester - Institutional Repository