Microhydration of Organophosphates: Ab Initio … · ab initio simulations of OP in vacuum, there...
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Mi Microhydration of Organophosphates: Ab Initio Investigations Todd M. Alam, Janelle Jenkins, C. J. Pearce, Cathy Clewett Sandia National Labs, Albuquerque, NM 87185 ABSTRACT Water is expected to play a crucial role in modulating the conformational energetics of organophosphates (OP). It is expected that these different energetics may be important in controlling the interaction, lifetime and reaction kinetics of OP in solution and at interface of surfaces. Unfortunately, while there have been extensive ab initio simulations of OP in vacuum, there are a limited number of computational studies that either implicitly or explicitly incorporate water into the investigations. Simulations involving water would provide structural and dynamical details of the OP-water complexes, along with the impact of hydration on the observed energies. Initial computational investigations have proven either inclusive or inconsistent when evaluating different OP. For example, simulations incorporating polarized continuum models (PCM) show that the conformational potential energy surface (PES) of Sarin is essentially unchanged from the PES obtained from simulations in vacuum. In contrast, simulations of the PES for dimethyl methyl phosphonate (DMMP) reveal distinct differences between the vacuum and PCM results. Incorporation of the PCM produced changes in the energies of the different equilibrium conformers, with the relative energies of the higher energy conformers (III and IV) being reduced by approximately 2 kcal mol -1 , in comparison to the vacuum results. This observation would suggest that DMMP structural conformations originally dismissed based on the energetics from vacuum simulations, may become important in discussions involving the adsorption and kinetics of hydrated DMMP. The PCM results are also commonly at odds to results and trends obtained from ab initio calculations that explicitly included water molecules complexed to the OP. Presently it is unclear at what point increasing the number of explicit water converges to the PCM results. To investigate the impact of water on conformational energetics, we report a series of microhydration simulations. A large number of initial starting conformation were explored for the Sarin•nH 2 O and DMMP•nH 2 O complexes (n = 1, 2, 3), followed by optimization to a local equilibrium conformation. It is shown that these OP•Water complexes are very rich in structural conformations and exhibit a distribution of energies. We will present the implementation of an ab initio “Evolutionary Generation Tree” method to characterize these different microhydrated clusters and to map out transitions between energetically favorable conformers. This work is supported by DTRA JSTO-CBD Proposal # CBS.FATE.03.10.SN.002. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy’s National Nuclear Security Administration. Partial student support (CJP) was provided under the DOE FaST program. CONCLUSIONS The addition of explicit waters in the hydration produces a rich variation in the energetics of both Sarin and DMMP. At higher hydration levels the presence of other conformers become energetically favorable. The structural perturbation are small for Sarin and DMMP. The adsorption energies for 2 explicit waters are on the order of the adsorption energy for SARIN and DMMP on SiOH surfaces. This suggest that micro-hydration of Sarin and DMMP will interfere with the SiO 2 surface adsorption. This work a part of a larger coordinated effort that includes: T17- 012, Ab Initio Studies into the Role of Water on the Energetics of Organophosphate Surface Adsorption, Janelle Jenkins (SNL), T14- 009 Molecular Simulations of Dimethyl Methylphosphonate Aqueous Solutions, Pratt (Tulane University). Microhydration Structures: Evolutionary Generation Tree SARIN A φ = 335° B φ = 166° Generation G0 Gas Phase Parent Structures G1 1 Water G2 2 Waters G3 3 Waters CPP 1 ,…..DPI, DIP, etc. A’PP 1 P 2 , ………. etc. C φ = 15° AP BP BI CPP 1 P 2 Parent Structure 1 st Water Binding Location 3rd Water Binding Location P – water bound to PO I – water bound to O-Isopropyl F – water bound to Fluorine Italic – water bound to another water in inner sphere. Superscript - identifies which inner- sphere water 2nd Water Binding Location The parent structures were determined using the dihedral angle of the P(1)-O(5)-C(6)-H(10) Bond. DMMP A B C Generation G0 G1 1 Water G2 2 Waters G3 3 Waters AM AP CP CM CPP,…..CPM, CMP APPP 1 , ………. APPP 1 Parent Structure 1 st Water Binding Location 2nd Water Binding Location 3rd Water Binding Location P – water bound to PO M – water bound to OMe Italic – water bound to another water in inner sphere. Superscript - identifies which inner-sphere water See potential energy scan for definition of structures Sarin Potential Energy Surface Dihedral (P-O-C-H) Degrees 0 100 200 300 ∆ E(kcal mol -1 ) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Gas Phase PCM Gas Phase + 1H 2 O PCM + 1H 2 O B A C Potential Energy Scan for DMMP Dihedral (O1-P1-O3-C3) 0 50 100 150 200 250 300 350 Dihedral (O1-P1-O2-C2) 0 50 100 150 200 250 300 0 2 4 6 8 10 12 A C B C A Potential Energy Scan for DMMP (PCM) Dihedral (O1-P1-O3-C3) 0 50 100 150 200 250 300 350 Dihedral (O1-P1-O2-C2) 0 50 100 150 200 250 300 0 2 4 6 8 A C B C A D D The impact of both implicit (PCM) and explicit waters on the conformational structure of Sarin is limited. The higher energy conformer B is further destabilized in present of the implicit PCM field. The addition of explicit waters results in a closely related A structure at 345 o . The impact of the implicit PCM solvent is significant for the DMMP conformational surface. The PCM stabilizes the relatively high energy C conformation with respect to A. It also results in a new conformational minima D, which is also observed in the presence of explicit waters. Conformational Energetics in Gas Phase and PCM PES evaluated using DFT B3LYP 6311++(2d,2p) basis set in Gaussian09 unless otherwise noted. 0.0 kcal 0.24 kcal 2.3 kcal A B C 0.47 kcal B,C A 0.0 kcal 0.2 kcal 2.8 kcal Gas Phase 1 Explicit H 2 O PCM (Implicit) Does the inclusion of explicit waters converge to the energy results of implicit PCM? If so, at what point does this occur? Are explicit waters required to describe the adsorption energetics and trends? Sarin Table B: Energies of Water Dimer Complex Energies H 2 O 2H 2 O E -76.4620395 H -152.93139931 E C -- -152.93136855 BSSE -- 6.246x10 -4 H (0.39 kcal mol -1 ) E C (Opt) -- -152.931376 δ deformation -- 6.9x10 -5 (0.043 kcal mol -1 ) δ HB -- -7.36x10 -3 (4.62 kcal mol -1 ) δ -- -7.30x10 -3 (4.58 kcal mol -1 ) E = Energies at DFT B3LYP/6-311++(2d,2p), E C = Corrected Energies for BSSE, δ deformation = deformation energy, δ HB = hydrogen bonding energy. The micro-hydration of Sarin shows only a small variation of ∆E(adsorption) with conformer. The addition of the 2nd and 3rd water to the hydration sphere produces a smaller change in the ∆E(adsorption). For 3 waters hydrated to Sarin the relative energies of the different conformation become intermingled. Hydrogen bonding between waters in the hydration sphere become the dominant energetic component with higher water content in comparison to the hydrogen bond to the P=O bond. There are subtle structural changes in the Sarin conformer with addition of explicit waters. Table A: Relative predicted energies (kcal/mol ) for gas phase conformers of DMMP optimized at B3LYP 6-311++G(2d,2p) in the gas phase. Conformer B3LYP MP2 B3LYP (PCM) MP2 (PCM) ∆G Solv (B3LYP) ∆G Solv (MP2) A 0.0 0 0.0 0.0 -6.4 -6.3 B 0.2 0.3 0.5 0.5 -6.1 -6.1 C 2.3 2.0 0.5 0.2 -8.2 -8.0 Table C: DMMP + 1H 2 O, Micro-Solvation Energetics Conformer E ∆ (kcal mol -1 ) a E Ads ∆ (kcal mol -1 ) E Opt Ads ∆ (kcal mol -1 ) conf DMMP δ (kcal mol -1 ) BSSE (kcal mol -1 ) Solv G ∆ (kcal mol -1 ) AP 0.00 [0.08 (0.09)] -7.70 [-7.51 (0.18)] -7.13 [-7.05 (0.09)] 0.44 [0.34 (0.09)] 0.48 [0.47 (0.02)] -7.52 [-7.14 (0.37)] AM 3.46 [3.55 (0.10)] -3.87 [-3.78 (0.11)] -3.67 [-3.59 (0.10)] 0.17 [0.16 (0.01)] 0.58 [0.57 (0.01)] -8.77 [-8.64 (0.13)] BP 0.21 [0.58 (0.52)] b -7.58 [-7.14 (0.63)] -7.16 [-6.79 (0.52)] 0.31 [0.25 (0.09)] 0.49 [0.47 (0.03)] -8.12 [-7.60 (0.74)] BM 4.04 [-- (--)] c -3.25 [--(--)] c -3.32 [-- (--)] c -0.09 [--(--)] c 0.52 [--(--)] c -9.22 [--(--)] c CP 2.28 [2.48 (0.18)] -7.48 [-7.29 (0.17)] -7.11 [-6.92 (0.18)] 0.74 [0.27 (0.14)] 1.11 [0.49 (0.17)] -9.44 [9.14 (0.18)] CM -- d -- d -- d -- d -- d -- d DP 3.34 [-- (--)] c -7.56 [--(--)] c -- e -- e 0.45 [--(--)] c -8.99 [--(--)] c DM 7.36 [-- (--)] c -3.58 [--(--)] c -- e -- e 0.64 [--(--)] c -10.72 [--(--)] c a Relative energies with respect to lowest AP+1H 2 O configuration. Values given are lowest [average (Std. Dev.)] based on analysis of 30 configurations. b Only 2 of this configuration observed. c Only a single example of this configuration observed. d No examples of this DMMP+1H 2 O configuration observed. e No stable gas phase conformation. DMMP
Transcript of Microhydration of Organophosphates: Ab Initio … · ab initio simulations of OP in vacuum, there...
Mi
Microhydration of Organophosphates: Ab Initio InvestigationsTodd M. Alam, Janelle Jenkins, C. J. Pearce, Cathy Clewett
Sandia National Labs, Albuquerque, NM 87185ABSTRACT
Water is expected to play a crucial role in modulating the conformational energetics of
organophosphates (OP). It is expected that these different energetics may be important in controlling the
interaction, lifetime and reaction kinetics of OP in solution and at interface of surfaces. Unfortunately,
while there have been extensive ab initio simulations of OP in vacuum, there are a limited number of
computational studies that either implicitly or explicitly incorporate water into the investigations.
Simulations involving water would provide structural and dynamical details of the OP-water complexes,
along with the impact of hydration on the observed energies. Initial computational investigations have
proven either inclusive or inconsistent when evaluating different OP. For example, simulations
incorporating polarized continuum models (PCM) show that the conformational potential energy surface
(PES) of Sarin is essentially unchanged from the PES obtained from simulations in vacuum. In contrast,
simulations of the PES for dimethyl methyl phosphonate (DMMP) reveal distinct differences between the
vacuum and PCM results. Incorporation of the PCM produced changes in the energies of the different
equilibrium conformers, with the relative energies of the higher energy conformers (III and IV) being
reduced by approximately 2 kcal mol-1, in comparison to the vacuum results. This observation would
suggest that DMMP structural conformations originally dismissed based on the energetics from vacuum
simulations, may become important in discussions involving the adsorption and kinetics of hydrated
DMMP. The PCM results are also commonly at odds to results and trends obtained from ab initio
calculations that explicitly included water molecules complexed to the OP. Presently it is unclear at what
point increasing the number of explicit water converges to the PCM results. To investigate the impact of
water on conformational energetics, we report a series of microhydration simulations. A large number of
initial starting conformation were explored for the Sarin•nH2O and DMMP•nH2O complexes (n = 1, 2, 3),
followed by optimization to a local equilibrium conformation. It is shown that these OP•Water complexes
are very rich in structural conformations and exhibit a distribution of energies. We will present the
implementation of an ab initio “Evolutionary Generation Tree” method to characterize these different
microhydrated clusters and to map out transitions between energetically favorable conformers.
This work is supported by DTRA JSTO-CBD Proposal # CBS.FATE.03.10.SN.002. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, awholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy’s National Nuclear Security Administration. Partial student support (CJP) wasprovided under the DOE FaST program.
CONCLUSIONS
The addition of explicit waters in the hydration produces arich variation in the energetics of both Sarin and DMMP.
At higher hydration levels the presence of other conformersbecome energetically favorable.
The structural perturbation are small for Sarin and DMMP.
The adsorption energies for 2 explicit waters are on theorder of the adsorption energy for SARIN and DMMP on SiOHsurfaces. This suggest that micro-hydration of Sarin andDMMP will interfere with the SiO2 surface adsorption.
This work a part of a larger coordinated effort that includes: T17-012, Ab Initio Studies into the Role of Water on the Energetics ofOrganophosphate Surface Adsorption, Janelle Jenkins (SNL), T14-009 Molecular Simulations of Dimethyl MethylphosphonateAqueous Solutions, Pratt (Tulane University).
Microhydration Structures: Evolutionary Generation TreeSARIN
Aφ = 335°
Bφ = 166°
Generation G0Gas Phase
Parent Structures
G11 Water
G22 Waters
G33 Waters
CPP1,…..DPI, DIP, etc. A’PP1P2, ………. etc.
Cφ = 15°
AP BP BI
CPP1P2
Parent Structure
1st WaterBinding Location
3rd WaterBinding Location
P – water bound to POI – water bound to O-IsopropylF – water bound to FluorineItalic – water bound to another water in inner sphere.Superscript - identifies which inner-sphere water
2nd WaterBinding Location
The parent structures were determined using the dihedral angle of the P(1)-O(5)-C(6)-H(10) Bond.
DMMP
A B C
Generation G0
G11 Water
G22 Waters
G33 Waters
AM AP CP CM
CPP,…..CPM, CMP APPP1, ……….
APPP1
Parent Structure
1st WaterBinding Location
2nd WaterBinding Location
3rd WaterBinding Location
P – water bound to POM – water bound to OMeItalic – water bound to another water in inner sphere.Superscript - identifies which inner-sphere water
See potential energy scan for definition of structures
Sarin Potential Energy Surface
Dihedral (P-O-C-H) Degrees0 100 200 300
∆� E
(kca
l mol
-1)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Gas PhasePCMGas Phase + 1H2OPCM + 1H2O
B
A
C
Potential Energy Scan for DMMP
Dihedral (O1-P1-O3-C3)0 50 100 150 200 250 300 350
Dih
edra
l (O
1-P1
-O2-
C2)
0
50
100
150
200
250
300
0 2 4 6 8 10 12
A
C
B C A
Potential Energy Scan for DMMP (PCM)
Dihedral (O1-P1-O3-C3)0 50 100 150 200 250 300 350
Dih
edra
l (O
1-P1
-O2-
C2)
0
50
100
150
200
250
300
0 2 4 6 8
A
C
B C A
D
D
The impact of both implicit(PCM) and explicit waters on theconformational structure of Sarinis limited.
The higher energy conformer Bis further destabilized in presentof the implicit PCM field.
The addition of explicit watersresults in a closely related Astructure at 345o.
The impact of the implicit PCM solvent is significant for the DMMP conformationalsurface.
The PCM stabilizes the relatively high energy C conformation with respect to A.
It also results in a new conformational minima D, which is also observed in thepresence of explicit waters.
Conformational Energetics in Gas Phase and PCM
PES evaluated using DFTB3LYP 6311++(2d,2p) basisset in Gaussian09 unlessotherwise noted.
0.0 kcal
0.24 kcal
2.3 kcal
A B
C
0.47 kcalB,C
A0.0 kcal
0.2 kcal
2.8 kcal
Gas Phase 1 ExplicitH2O
PCM (Implicit)
Does the inclusion of explicit waters converge to the energy results of implicit PCM? If so, at what point does this occur?Are explicit waters required to describe the adsorption energetics and trends?
Sarin
Table B: Energies of Water Dimer Complex
Energies H2O 2H2O E -76.4620395 H -152.93139931 EC -- -152.93136855 BSSE -- 6.246x10-4 H (0.39 kcal mol-1) EC(Opt) -- -152.931376 δdeformation -- 6.9x10-5 (0.043 kcal mol-1) δHB -- -7.36x10-3 (4.62 kcal mol-1) δ -- -7.30x10-3 (4.58 kcal mol-1)
E = Energies at DFT B3LYP/6-311++(2d,2p), EC = Corrected Energies for BSSE, δdeformation = deformation energy, δHB = hydrogen bonding energy.
The micro-hydration of Sarin shows only a smallvariation of ∆E(adsorption) with conformer.
The addition of the 2nd and 3rd water to the hydrationsphere produces a smaller change in the∆E(adsorption).
For 3 waters hydrated to Sarin the relative energies ofthe different conformation become intermingled.
Hydrogen bonding between waters in the hydrationsphere become the dominant energetic componentwith higher water content in comparison to thehydrogen bond to the P=O bond.
There are subtle structural changes in the Sarinconformer with addition of explicit waters.
Table A: Relative predicted energies (kcal/mol ) for gas phase conformers of DMMP optimized at B3LYP 6-311++G(2d,2p) in the gas phase.
Conformer B3LYP MP2 B3LYP (PCM) MP2 (PCM) ∆GSolv (B3LYP)
∆GSolv (MP2)
A 0.0 0 0.0 0.0 -6.4 -6.3 B 0.2 0.3 0.5 0.5 -6.1 -6.1 C 2.3 2.0 0.5 0.2 -8.2 -8.0
Table C: DMMP + 1H2O, Micro-Solvation Energetics
Conformer E∆
(kcal mol-1)a EAds∆
(kcal mol-1) EOpt
Ads∆
(kcal mol-1)
confDMMPδ
(kcal mol-1)
BSSE (kcal mol-1) SolvG∆
(kcal mol-1) AP 0.00
[0.08 (0.09)] -7.70 [-7.51 (0.18)]
-7.13 [-7.05 (0.09)]
0.44 [0.34 (0.09)]
0.48 [0.47 (0.02)]
-7.52 [-7.14 (0.37)]
AM 3.46 [3.55 (0.10)]
-3.87 [-3.78 (0.11)]
-3.67 [-3.59 (0.10)]
0.17 [0.16 (0.01)]
0.58 [0.57 (0.01)]
-8.77 [-8.64 (0.13)]
BP 0.21
[0.58 (0.52)]b -7.58 [-7.14 (0.63)]
-7.16 [-6.79 (0.52)]
0.31 [0.25 (0.09)]
0.49 [0.47 (0.03)]
-8.12 [-7.60 (0.74)]
BM 4.04 [-- (--)]c
-3.25 [--(--)]c
-3.32 [-- (--)]c
-0.09 [--(--)]c
0.52 [--(--)]c
-9.22 [--(--)]c
CP 2.28
[2.48 (0.18)] -7.48 [-7.29 (0.17)]
-7.11 [-6.92 (0.18)]
0.74 [0.27 (0.14)]
1.11 [0.49 (0.17)]
-9.44 [9.14 (0.18)]
CM --d --d --d --d --d --d DP 3.34
[-- (--)]c -7.56 [--(--)]c
--e
--e 0.45
[--(--)]c -8.99 [--(--)]c
DM 7.36 [-- (--)]c
-3.58 [--(--)]c
--e --e 0.64 [--(--)]c
-10.72 [--(--)]c
a Relative energies with respect to lowest AP+1H2O configuration. Values given are lowest [average (Std. Dev.)] based on analysis of 30 configurations. b Only 2 of this configuration observed. c Only a single example of this configuration observed. d No examples of this DMMP+1H2O configuration observed. e No stable gas phase conformation.
DMMP
