Electrochemical Surface Science - American Chemical Society

Chapter 23

Infrared Spectroelectrochemistry of Surface Species In Situ Surface Fourier Transform Infrared Study of Adsorption of Isoquinoline at a Mercury Electrode 1

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Daniel Blackwood , Carol Korzeniewski , William McKenna , Jianguo Li , and Stanley Pons 1

Downloaded by TUFTS UNIV on June 3, 2018 | https://pubs.acs.org Publication Date: November 11, 1988 | doi: 10.1021/bk-1988-0378.ch023

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Department of Chemistry, University of Utah, Salt Lake City, UT 84112 Department of Chemistry, University of Michigan, Ann Arbor, MI 48109

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Subtractively normalized i n t e r f a c i a l Fourier transform i n f r a r e d spectroscopy (SNIFTIRS), has been used extensively to examine interactions of species at the electrode/electrolyte interface. In the present work, the method has been extended to probe interactions at the mercury s o l u t i o n i n t e r f a c e . The diminished p o t e n t i a l dependent frequency s h i f t s of species adsorbed at mercury electrodes are compared with s h i f t s observed for s i m i l a r species adsorbed at d-band metals. Several spectroscopic techniques have been developed for the i n v e s t i g a t i o n of electrode - s o l u t i o n i n t e r f a c i a l phenomena (1-7). The i n f r a r e d techniques include ( i ) s u b t r a c t i v e l y normalized i n t e r f a c i a l Fourier transform i n f r a r e d spectroscopy, (SNIFTIRS) (7) i n which i n f r a r e d spectra are c o l l e c t e d at two d i f f e r e n t p o t e n t i a l s and a difference spectrum obtained by subtraction and normalization, ( i i ) electrochemically modulated i n f r a r e d spectroscopy, EMIRS (4), a dispersive spectrophotometric technique i n which the electrode p o t e n t i a l i s modulated at a set frequency and the r e s u l t i n g attenuation of the r e f l e c t i n g i n f r a r e d r a d i a t i o n i s analyzed by phase s e n s i t i v e detection, and ( i i i ) i n f r a r e d r e f l e c t i o n absorption spectroscopy, IRRAS (6), i n which the p o l a r i z a t i o n of the incident i n f r a r e d r a d i a t i o n i s modulated at a high rate (ca. 74 kHz) between the s- and the p- states to d i s t i n g u i s h between adsorbed and s o l u t i o n dissolved i n f r a r e d absorbers. The attenuated i n f r a r e d s i g n a l i s again monitored with the a i d of phase s e n s i t i v e detection techniques. The c h a r a c t e r i s t i c s of the two types of spectra are l i s t e d i n Table I. The s e l e c t i o n rules that determine i n t e r a c t i o n of i n f r a r e d r a d i a t i o n with adsorbates are w e l l known, and a r i s e from the differences i n the r e l a t i v e i n t e n s i t i e s of the s- and p-polarized components of the electromagnetic f i e l d vectors of the incident i n f r a r e d r a d i a t i o n at the metal-solution interface. The s-polarized component undergoes a phase s h i f t upon r e f l e c t i o n from a metal surface which r e s u l t s i n zero f i e l d strength at the surface, while the p-component f i e l d strength increases at the surface as the angle 0097-6156/88/0378-0338$06.00/0 * 1988 American Chemical Society

Soriaga; Electrochemical Surface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

B L A C K W O O D ET AL.

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Table I.

Infrared Spectroelectrochemistry of Surface Species

A Comparison of the C h a r a c t e r i s t i c s of Infrared V i b r a t i o n a l Bands A r i s i n g from Bulk Solution Species and Adsorbed Species

BULK SOLUTION SPECIES Bands present with either si) or p-polarized l i g h t . i i ) Band positions independent of ii) potential. i i i ) Relative i n t e n s i t y of bands i i i ) independent of p o t e n t i a l . i v ) Normally IR inactive bands iv) not observed.


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ADSORBED SPECIES Bands present only with p-polarized l i g h t . Band positions may s h i f t with p o t e n t i a l . Relative i n t e n s i t y may change with p o t e n t i a l . Some may be observed.

of incidence increases. As a r e s u l t , only p-polarized l i g h t can i n t e r a c t with species which are either adsorbed onto or are very close to the electrode surface. In addition, the normal modes of an adsorbate that i n t e r a c t with the r a d i a t i o n must have a component of the derivative of the dipole moment (with respect to the normal coordinate) perpendicular to the metal surface. In addition, modes of an adsorbate activated by v i b r o n i c coupling to the surface may i n t e r a c t with the r a d i a t i o n (vide i n f r a ) . P o t e n t i a l dependence of the frequencies of i n f r a r e d bands a r i s e because of the change i n extents of bonding of the adsorbate to the metal surface as the p o t e n t i a l i s changed. The e f f e c t has been explained by several models (vide i n f r a ) . To i l l u s t r a t e the p o t e n t i a l dependence of adsorbed species, the SNIFTIRS spectra of the b 3 r i n g bending mode of p-difluorobenzene a t a platinum electrode i s i l l u s t r a t e d i n Figure 1 (8) The upward pointing band i s c l e a r l y independent of p o t e n t i a l and i s assigned to the s o l u t i o n species whereas the p o s i t i o n of the downward pointing band s h i f t s monotonically (at constant i o n i c strength) with p o t e n t i a l and i s therefore due to an adsorbed species. The p a r t i c u l a r system investigated i n t h i s work was the adsorption of isoquinoline at a mercury electrode, which has been previously studied by several techniques, including e l e c t r o c a p i l l a r y measurements (9), ellipsometry (10), double layer capacity measurements (11), and a range of p o t e n t i a l step techniques (12-15). The i n t e r e s t i n t h i s system i s due i n part to the f a c t that one observes w e l l defined t r a n s i t i o n s i n the measurements as the adsorbed molecules undergo t r a n s i t i o n s i n surface o r i e n t a t i o n and packing density under c e r t a i n experimental conditions. Isoquinoline has been shown to be adsorbed on mercury i n four d i f f e r e n t orientations (Figure 2). The previous investigations indicate the following behavior for the isoquinoline o r i e n t a t i o n as a function of p o t e n t i a l and concentration: at low negative potentials and low bulk concentrations, the molecules are believed to l i e f l a t on the mercury surface (molecular plane p a r a l l e l to the surface). On increasing e i t h e r the p o t e n t i a l ( i n the negative d i r e c t i o n ) or the bulk concentration, the isoquinoline molecules are forced up into either the 4,5 p o s i t i o n (10) or the 5,6 p o s i t i o n (9). This r e o r i e n t a t i o n occurs gradually with the changing coordinates, and proceeds through a series of phases containing mixtures of these three orientations of isoquinoline molecules. u



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ELECTROCHEMICAL SURFACE SCIENCE

Figure 1. SNIFTIRS difference spectra of the D3 r i n g bending mode of p-difluorobenzene at a platinum electrode as a function of modulation p o t e n t i a l . U


BLACKWOOD ETAL.

Infrared Spectroekctrochemistry of Surface Species


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Figure 2. L i k e l y orientations f o r the adsorption of isoquinoline on mercury.


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ELECTROCHEMICAL SURFACE SCIENCE

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Increases i n the p o t e n t i a l to more negative values (at s u f f i c i e n t l y high concentrations) r e s u l t s i n an abrupt r e o r i e n t a t i o n to the 6,7 p o s i t i o n . The reason that t h i s second t r a n s i t i o n i s much sharper than the f i r s t l i e s i n the f a c t that mixed phases which would contain the 6,7 o r i e n t a t i o n are e n e r g e t i c a l l y less favorable than a complete monolayer of any of the pure standing o r i e n t a t i o n a l phases. G i e r s t et a l (9) have produced a graph showing the dependence of the s u p e r f i c i a l excess on both p o t e n t i a l and bulk concentration from t h e i r e l e c t r o c a p i l l a r y data; we reproduce some of t h e i r data i n Figure 3. For an isoquinoline molecule adsorbed onto the surface of mercury, the component of i t s t o t a l dipole moment that i s perpendicular to the surface w i l l increase as i t s o r i e n t a t i o n changes from: Flat - 4 , 5 - 5 , 6 - 6 , 7 As the isoquinoline molecule reorients i n the order l i s t e d above, the absorption of i n f r a r e d r a d i a t i o n by the in-plane v i b r a t i o n a l modes would be expected to increase, while that of the out-of-plane modes would be predicted to decrease ( i n accordance with the surface s e l e c t i o n r u l e as described above). In the f l a t o r i e n t a t i o n there i s no component of the dipole moment perpendicular to the surface f o r the in-plane modes, and under the surface s e l e c t i o n r u l e these modes w i l l not be able to absorb any of the incident r a d i a t i o n . However, as mentioned above, i n f r a r e d active modes (and i n some cases i n f r a r e d forbidden t r a n s i t i o n s ) can s t i l l be observed due to f i e l d - i n d u c e d v i b r o n i c coupled i n f r a r e d absorption (16-20). We have determined that t h i s type of i n t e r a c t i o n i s present i n t h i s p a r t i c u l a r system. EXPERIMENTAL Isoquinoline ( A l d r i c h 97%) was p u r i f i e d by r e f l u x i n g with BaO f o r 30 minutes and d i s t i l l i n g i n vacuo. The r e s u l t i n g white c r y s t a l l i n e s o l i d had a melting point of 26°C. The p u r i f i e d isoquinoline was stored i n the dark, at 0°C and under an argon atmosphere. Mercury was t r i p l y d i s t i l l e d (American S c i e n t i f i c ) and a l l other chemicals were of AnalaR grade. Solutions were prepared with t r i p l y d i s t i l l e d water. Glassware was cleaned i n a 50:50 (v:v) mixture of HNO3 and H2SO4, and rinsed and steamed ( t r i p l y d i s t i l l e d water) f o r h a l f an hour. A t h i n layer c e l l was designed (Figure 4) which could be mounted v e r t i c a l l y on top of the sample chamber of the spectrometer. The mercury was held i n p o s i t i o n by a glass tube, and e l e c t r i c a l contact was achieved with a piece of platinum wire inserted into the mercury. P o t e n t i a l s are measured with respect to a saturated calomel electrode (SCE). The technique used to acquire the data i n t h i s paper was SNIFTIRS. A schematic diagram of the required apparatus i s shown i n Figure 5, and has been described i n d e t a i l elsewhere. The FTIR spectrometer used was a vacuum bench Bruker IBM Model IR/98, modified so that the o p t i c a l beam was brought upwards through the sample compartment and made to r e f l e c t from the bottom of the h o r i z o n t a l mercury surface. The methods used herein are adapted from a configuration that has been used by Bewick and co-workers (21) at Southampton.


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Infrared Spectroelectrochemistry ofSurface Species

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Electrochemical Surface Science - American Chemical Society

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