Hydrogen Bond Dynamics in Concentrated Salt SolutionsHydrogen Bond Dynamics in Concentrated Salt SolutionsUltrafast Two Dimensional Infrared Vibrational Echo ExperimentsUltrafast Two Dimensional Infrared Vibrational Echo Experiments

Michael D. Fayer, Stanford University, DMR 0652232

Water hydrogen bond dynamics play a central role in many processes in chemistry, biology, materials science, and geology. Frequently, water has salts in it. A fundamentally important question is how do charges influence the dynamics of water? We have used ultrafast two-dimensional infrared (2D-IR) vibrational echo spectroscopy to study hydrogen bond dynamics of water in NaBr solutions. 2D-IR vibrational echo spectroscopy is akin to 2D NMR, but it operates on ultrafast time scales. By using very concentrated NaBr solutions, we observe how water molecules behave in the presence of ions. The data and curves in the plot are a representation of the time evolution of the 2D spectra. A 2D spectrum is shown in the inset. As the NaBr concentration increases, the decays slow, which shows that the rearrangement of the hydrogen bond structure slows because more and more hydrogen bonds are made to ions rather than solely among water molecules. The concentrations of 1.5 M, 3.0 M, and 6 M NaBr correspond to 32, 16, and 8 water molecules per NaBr, respectively. The time constants for the complete randomization of the hydrogen bond structure (breaking and making new hydrogen bonds) are given in the figure. Remarkably, even for 6 M (less than one solvation shell of water per ion) the time slows by less than a factor of 3 compared to pure water.

BrNa+

NaBr in WaterNaBr in Water

0 1 2 3 4 5 6 7 8 9 100.0

0.1

0.2

0.3

0.4

0.5

0.6

C = 6.0 M = 4.8 psC = 3.0 M = 3.5 psC = 1.5 M = 2.6 pspure water = 1.7 ps

Tw (ps)

CC

LS

(Tw)

Tw = 0.2 ps

m(c

m- 1

)

(cm-1)

2700

2600

2500

2400

260025002400

Tw = 0.2 ps

m(c

m- 1

)

(cm-1)

2700

2600

2500

2400

260025002400

0 1 2 3 4 5 6 7 8 9 100.0

0.1

0.2

0.3

0.4

0.5

0.6

C = 6.0 M = 4.8 psC = 3.0 M = 3.5 psC = 1.5 M = 2.6 pspure water = 1.7 ps

Tw (ps)

CC

LS

(Tw)

Tw = 0.2 ps

m(c

m- 1

)

(cm-1)

2700

2600

2500

2400

260025002400

Tw = 0.2 ps

m(c

m- 1

)

(cm-1)

2700

2600

2500

2400

260025002400

Tw = 0.2 ps

m(c

m- 1

)

(cm-1)

2700

2600

2500

2400

260025002400

Tw = 0.2 ps

m(c

m- 1

)

(cm-1)

2700

2600

2500

2400

260025002400

Hydrogen Bond Dynamics in Concentrated Salt SolutionsHydrogen Bond Dynamics in Concentrated Salt SolutionsUltrafast Two Dimensional Infrared Vibrational Echo ExperimentsUltrafast Two Dimensional Infrared Vibrational Echo Experiments

Michael D. Fayer, Stanford University, DMR 0652232

Education: A post doc and two graduate students (Sungnam Park, David Moilanen, and Kyungwon Kwak) contributed to this work. All three of these students are exceedingly talented. They have developed skills in complex experimental system design and implementation, and computer programming for control of experiments and for mathematical modeling of physical phenomena. Of most importance, they learn to combine skills to attack scientific problems. All three are continuing work in the Fayer labs at Stanford on this problem and related problems. Of particular interest is the extension of 2D-IR vibrational echo experiments and other methods to the study of room temperature organic ionic liquids.

Outreach: The PI wrote a proposal, which was funded, to the Dreyfus Foundation to augment an existing NSF program at Stanford that brings high school science teacher to campus over the summer. The Dreyfus Foundation grant funded three teachers to participate in the program this summer. The teachers worked in research laboratories to give them familiarity with modern research methods and the application of the scientific method to the solution of current problems. The teachers also took training that provides materials and techniques for classroom science instruction. The teachers are:Annie Pang, teaches Chemistry at College Park High School, Mount Diablo School District (in Prof. J. Kohler's lab in Chemistry).Steven Chow, teaches Biology, Bio Tech, and Physical Science at Washington High School, Fremont Union School District (in Prof. S. Kim's lab, Developmental Biology).John Servanda, teaches Biology and physical sciences at the Boeger Middle School, Mount Pleasant School District (in Prof. W.E. Moerner's lab, Chemistry).