| Author | Message | ||
     Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 138 Registered: 9-2001 |
If you want the "true" spectrum of water, then measurement in the gas phase "diluted" in argon isn't good enough, either, because the water spectrum will be perturbed by collisions of the water molecules with the argon atoms. The effect of pressure on the measurement of gases is to broaden the rotational bands and cause overlapping of the vibration-rotation structure; this effect is well-known in mid-IR spectroscopy (in fact, I have some mid-IR spectra of methane that show the effect very nicely), and see no reason to believe it won't affect near-IR spectra, too. The way it's overcome in mid-IR work is to measure the pure gas (water vapor, in this case) at reduced pressure; at a pressure low enough that interactions between water molecules are few and far enough between not to affect the spectrum. How low is that? You have to determine it experimentally, by measuring the spectrum at lower and lower pressures until it stops changing. Of course, that also means that the "concentration" of gas (in terms of molecules per unit volume) keeps decreasing, and along with it the absorbance. Since NIR bands are weak to begin with, this means you have to have a VERY long pathlength. Special gas cells containing mirrors to bounce the light back and forth through the sample many times are available for such measurements, but as fas as I know, nobody has ever used them for measuring NIR spectra. It may be that even those don't give a large enough pathlength to allow measurement of the NIR spectra. \o/ /_\ | ||
| Gavriel Levin (levin) Senior Member Username: levin Post Number: 55 Registered: 1-2006 |
Just a small possible contribution from a truly non-expert on the theoretical side of the vibrational spectroscopy - in the liquid form water absorption shows very wide "peaks" primarily because there is not even one O-H bond that is not affected by the neighbouring molecules through the hydrogen bonding due to the very high polarity of the molecule. To measure the "true" spectrum of water will be possible in a vapor phase, where water molecules will be "dissolved" in say Argon carrier gas, thus "separating" the water molecules, and with a long pathlength, it will be possible to measure the "net" absorptions. Thanks, Gabi Levin | ||
| Kenneth Gallaher (ken_g) Senior Member Username: ken_g Post Number: 29 Registered: 7-2006 |
A key question is what do you plan to do with this information? Water is a highly ambiguous beast, environment: temperature, other components matter in a huge way. For the real world "water" will change it's definition whenever anything changes. | ||
| Tony Davies (td) Moderator Username: td Post Number: 165 Registered: 1-2001 |
Hello Lois and SGBK, What has not been mentioned so far, but it really needs to be remembered, is that water is very strongly hydrogen bonded and you may not see any �pure� vibrations at normal temperatures In order to see these you need to measure dilute solutions of water in a non-hydrogen bonding solvent. Chloroform was the classic choice but it may not be available in most labs these days! Best wishes, Tony | ||
| SGBK (sgbk) New member Username: sgbk Post Number: 3 Registered: 4-2006 |
Dear Dr. Weyer, An understanding of the nomenclature is precisely the problem with me. From what I understand (and you mention this too), v1,3+v2 should either be v1+v2 or v3+v2 and 2v1,3 should either be 2v1 or 2v3; and I also understand that the band could also be v1+v3. My only question is whether we can do away with all this uncertainty? As for the authors' names, here is the info: Vibrational Spectroscopy 1994, 7, 243 ~ Fred O. Libnau, Olav M Kvalheim, Alfred A Christy, Jostein Toft. Title:Spectra of water in the near- and mid-infrared region and Canadian J Chem 1966, 44, 1699 ~ Michael Falk, T A Ford Title: Infrared Spectrum and Structure of Liquid Water Many thanks for your response. | ||
| Lois Weyer (lois_weyer) Advanced Member Username: lois_weyer Post Number: 22 Registered: 7-2002 |
There may be no conflicts here. We just need to understand the nomenclature a little better. 2v1,3 is ambiguous. v1 is the symmetric vibration and v3 the asymmetric. The 7000 cm-1 band can't be twice both the symmetric and the asymmetric as that would come out to 14000 cm-1. I think it is generally accepted that the 7000 cm-1 band is the sum of v1 and v3. Likewise, what does v1,3 + v2 mean? It must be either v1 + v2 or V3 + v2, and I think it is generally accepted that the 5200 cm-1 band is v3 + v2. Alternately, perhaps your references are saying that v1 and v3 are coupled? Could you provide the author's names please? | ||
| SGBK (sgbk) New member Username: sgbk Post Number: 2 Registered: 4-2006 |
Hello again, Thank you for your response, and I have sent you an email for the reference you wrote about. Good that you brought up the 1450 and 1940 bands. I think I need some guidance on this as well. The article Vibrational Spectroscopy 1994, 7, 243 (Spectra of water in the near- and mid-infrared region) mentions this about the aforementioned bands. Pg 247: "Following the interpretation of the overtone bands of Luck [ref], the combination of the OH-stretching band and the OH bending band, v1,3 + v2 is found at approximately 5200 cm-1, the first overtone of the OH stretching band 2v1,3 is found around 7000 cm-1,..." And the article Can. J. Chem. 1966, 44, 1699 has a figure on Pg 1701 which has the band assignments in water. Can someone please guide me on the unambiguous assignemnt of v1 and v3 stretching modes of water in the NIR region? Many thanks. | ||
| Lois Weyer (lois_weyer) Intermediate Member Username: lois_weyer Post Number: 21 Registered: 7-2002 |
The band at approximately 1780 nm is said to be a combination of nu2, nu3, and an "intermolecular mode". Nu2 is bending and nu3 is the asymmetric stretch. The 1450 band, by the way, is not exactly an overtone. It is a combination of the symmetric and the asymmetric stretching. The 1940 band is the combination of bending and asymmetric stretching, not symmetric stretching. If you give me your email address I will send you original reference information on these. | ||
| SGBK (sgbk) New member Username: sgbk Post Number: 1 Registered: 4-2006 |
Hello, I have a question concerning the NIR spectrum of water. The spectrum shows a shoulder around ~1800 nm in between the first overtone of the OH sym. stretch and the (OH sym str+OH combi) band. Can someone please help me with an appropriate spectral assignment for this? Many Thanks. |