| Author | Message | ||
     Charles E. Miller (millerce) Member Username: millerce Post Number: 12 Registered: 10-2006 |
Tony, and everybody! I�m sure that this is WAY beyond the scope of the original request on this thread, but I promised you my recollection on combination bands- so here goes�. � Combination modes involve a simultaneous increase in the vibrational quantum number of two or more vibrational modes from the absorption of a single photon � These should not be confused with coupled vibrational modes, which are vibrations that involve more than one chemical bond (the classic examples of which are the �amide I� and �amide II� vibrations, which involve simultaneous vibrations of C=O, C-N and N-H bonds) � The presence of combination bands requires both mechanical anharmonicity (potential energy as a function of displacement has terms of higher order than the �harmonic� squared term) and electrical anharmonicity (dipole moment is a non-linear function of displacement); these allow vibrational modes to interact with one another � Combinations of fundamentals with overtones is possible, as well as combinations involving more than 2 vibrations � However, in order to �combine�, the vibrations must a) involve the same functional group, and b) have the same symmetry properties, per Group Theory. This can get interesting, because the symmetry properties of an overtone do not necessarily match those of the corresponding fundamental. � The intensity of combination bands depends on both the degree of anharmonicity of the participating vibrations and the change in dipole moment associated with the vibrations (for fundamental bands, only the latter influences band intensity) � Resonance effects (Fermi and Darling-Dennison), caused by interactions of overtone and combination modes that are made possible by anharmonicity, tend to complicate NIR spectra for lower-energy (lower frequency) overtones and combination bands � However, the spectra for higher-order overtone and combination bands tend to be less complicated, due to either the �local mode phenomenon� or less resonance due to less-favorable symmetry properties (there was some debate about this when I wrote the chapter�) Now I recall how frustrated I was trying to compile content on combination bands, as there was really not a lot of discussion of them in the literature. I had wanted to explain them from a classical mechanics viewpoint, to better convey them to non-spectroscopists � but found that I really had to fall back on quantum theory and group theory instead. Perhaps future authors will pursue an explanation based purely on classical mechanics (??..) By the way, there is one other useful reference that I did not mention previously: G. Herzberg, �Molecular Spectra and Structure� (1950). Best Regards, Chuck | ||
| Tony Davies (td) Moderator Username: td Post Number: 196 Registered: 1-2001 |
Chuck, Looking forward to your reflections! Tony | ||
| Charles E. Miller (millerce) Junior Member Username: millerce Post Number: 9 Registered: 10-2006 |
Greetings Tony! No worries! Unfortunately, I don't have access to many of these works anymore. This, combined with the fact that this takes me back a few years, means I'll need a bit of time to respond to your statements on combination bands. Time to break open some old textbooks... Regards, Chuck | ||
| Tony Davies (td) Moderator Username: td Post Number: 195 Registered: 1-2001 |
Hi Chuck! My apologies, I forgot that you had written the up-date on chapter 2 in W&N. Do you agree with my statements on combinations? I'm not sure where they come from; possibly combinations of discussions with Norman Jones, Peter Griffiths, Bill Fateley, Heinz Siesler et al., I do not have the Weyer and Workman book. What do they have on combinations? Best wishes, Tony | ||
| Charles E. Miller (millerce) Junior Member Username: millerce Post Number: 8 Registered: 10-2006 |
If you have access to the 2nd edition of the Williams and Norris "Near Infrared Technology.." Text, Chapter 2 addresses the various principles behind overtone and combination bands, using both classical and quantum mechanical constructs. In compiling this chapter, I relied heavily on some prior work, including: - R. Chang, "Basic Principles of Spectroscopy" (1971) - N. Colthup,et al "Introduction to Infrared and Raman Spectroscopy" (1990) - J.L. Duncan, Spectrochim Acta., 47A(1) (1991), and - Pimental and McClellan, "The Hydrogen Bond" (1960) Best Regards, Chuck | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 245 Registered: 9-2001 |
Another thought occurs to me: if you have time and patience, you can go back and look, starting from that time frame, at the annual Analytical Chemistry "Fundamental Reviews". There's likely one (or more than one) to have been printed. As I said, it was a "hot" topic at the time. \o/ /_\ | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 244 Registered: 9-2001 |
Well, when I was in school, the "bibles" of structure-spectra relations were by Bellamy and Nakanishi. That's all I remember for sure at this point. That was the "hot" topic at the time. I'm sure they included combination bands, but since they only measured in the mid-IR, those are the combinations (of the lower-frequency vibrations, e.g., -CN, -CO, -OO-, etc.) But I haven't kept up or looked at potential newer work for 35 years, at least! The only recent book that I can think of offhand is the one by Jerry Workman andd & Lois Weyer. \o/ /_\ | ||
| Tony Davies (td) Moderator Username: td Post Number: 194 Registered: 1-2001 |
Howard, Yes, I accept your reservations but in my opinion combination bands are generally not well discussed in the literature. Do you think that a comprehensive account has been published? Best wishes, Tony | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 243 Registered: 9-2001 |
Tony - I don't disagree with you, but as you point out, there's always a conflict between completeness, accuracy, and the possibility of overwhelming the newcomer by throwing too much material at him at once. In this case, I set the threshold level to "maximum simplicity" in my discussion. It's certainly appropriate, one Iyas has the basics under his belt, to start with the more complicated cases, but I think we still need to be careful not to overwhelm him. That's why I preferred to recommend that he go to the literature, so he could study the material at a pace that's convenient to him, and not become overwhelmed. \o/ /_\ | ||
| Tony Davies (td) Moderator Username: td Post Number: 193 Registered: 1-2001 |
HI, I just noticed that the word "stretch" is missing from the sentence: H-C-H bonds have two vibrations Should be: H-C-H bonds have two stretch vibrations .., Sorry! Best wishes, Tony | ||
| iyas (iyas) Advanced Member Username: iyas Post Number: 25 Registered: 7-2007 |
DEAR ALL thanks very much for your answer idid not reliease my simple questions will give this too overwelcomed answers and responses thanks mikael - also kind prof howard and prof donald burns and Donald J Dahm thanks venkatarman and for Tony Davies special thanks kind regards | ||
| Tony Davies (td) Moderator Username: td Post Number: 192 Registered: 1-2001 |
Hello Iyas, Howard, Don et al., I saw this interesting discussion flashing by but I have been too busy to join in. While I agree (unsusally) with almost everything that Howard has said I would like to expand on it. (The area of disagreement is in the statement "higher overtones are too weak to be observed". The fifth overtone of a benzene C-H stretch has been reported in the UV region. So it depends on the case being considered). Combination bands are more complex 1) H-C-H bonds have two vibrations; symmetric and anti-symmetric these have slightly different energy requirements so they appear as two fundemental vibrations in the mid-IR and both give rise to a series of overtones. They can also give rise to combinations bands with bend fundementals but additionally with themselves! You might have expected a single overtone peak but in fact you get three, which may not be separated. 2) Combinations are not restricted to pairs of bonds. Multiple bonds can be involved. This can be observed with a single stretch overtone and a series of combinations with 1, 2, 3 ... bend vibrations. 3) Combinations occur from the absorption of a photon of NIR energy being temporally absorbed by one of the partners before being shared with the other member(s) of the combination. This means that only this first bond is required to be infrared active. Other bonds could be Raman active or completely inactive. It is difficult to know where these absorptions will occur. 4) Stretch-bend combination are usually more intense that the corresponding first overtone of the stretch. The 1940 band in the NIR spectrum of water being the most famous example. Although combinations are observed in the NIR region what has happened is that two (or more) ground-state vibrations have been raised to the fundemental state but (because of coupling the energy requirement are slightly different) they are allowed to occur. When you consider all these possibilites you can see why NIR spectra become so complex except that many absorption are not resolved and you just see very broad areas of absorptions. Howard - I realise that you wanted to keep it simple but Iyas needs to know that there is quite a bit more to the story! Best wishes Tony | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 242 Registered: 9-2001 |
Just happened to come across a mention of this book, which looks like it would have information you want: "The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules" I'm sure it won't be hard to find it on amazon.com or on barnesandnoble.com \o/ /_\ | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 241 Registered: 9-2001 |
Venky - I should have pointed out that the book by Workman and Weyer, that I cited before, also has some discussion about the effects of intra-molecular changes on the spectrum. But for the extensive discussion that you're looking for, you'll still need to go to the classic mid-IR work on the relationship between spectra and the underlying chemical structures. \o/ /_\ | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 240 Registered: 9-2001 |
Venky - Don's answers are correct. These issues are not discussed very much in the NIR literature. They are, however, discussed at great length in the mid-IR literature, and if you want to find out more about the effects of the molecular environment, both the internal and external environments of a molecule, on its spectrum, that's where you need to look. The reference Don gave to the NIR handbook is a good starting point, however. To expand on Don's comments somewhat, we can note the following: 1) A harmonic oscillator corresponds to a parabolic energy envelope. A quantized harmonic oscillator (if one existed) would have equally spaced energy levels. In this case, a change of +/-1 in the quantum levels would correspond to the fundamental absorbance. A change of �2 in the quantum levels would correspond to the first overtone, and its frequency would be exactly twice that of the fundamental. 2) A non-harmonic oscillator, as Don said, corresponds to an asymmetric energy envelope; I'm not going to try to describe the shape of this envelope, you'll have to look at the diagram in the handbook to see what it's like. In any case, the important feature of this asymmetric energy envelope, that corresponds to the way atoms in a molecule actually vibrate, is that the quantum levels ae no longer at equally spaced energy levels; the energy difference between quantum levels decreases as the levels increase. Thus the difference in energy between states that are two quantum levels apart is slightly less than twice the difference of a change from the ground level to the first quantum level. Thus the absorbance band corresponding to this overtone vibration has a slightly lower frequency, and thus a slightly longer wavelength, than a simple integer calculation predicts. The actual energy levels, and thus the vibrational frequencies, depend on the structure of the molecules involved, including any substituents that may be present. This is the reason you see differences between the spectra of different materials, or the same material in different environments. Changes in the NIR spectrum reflect changes in the fundamental vibrations of the molecule, which can be observed in the mid-IR spectrum. If a chlorine atom (a highly electro-negative atom), for example, is substituted for a hydrogen atom, its electron-withdrawing characteristic reduces electron density in nearby bonds, thus weakening them, and causing their vibrations to shift to lower frequencies, corresponding to longer wavelengths. Other substituents atoms that are electropositive, can shift the vibrations of nearby atoms to higher frequencies. It gets very complicated. You may want to take a course on structure and mechanism in organic chemistry, if you really want to understand some of these effects. The vibrational frequencies are also affected by interactions with extraneous molecules. The most common example of this effect, that we see, is the effect of hydrogen bonding when something is dissolved in water, or simply has some water near it. \o/ /_\ | ||
| Donald J Dahm (djdahm) Advanced Member Username: djdahm Post Number: 24 Registered: 2-2007 |
The environment (neighboring molecules) of a molecule will affect the frequencies at which a partiular bond vibrates. As far as Howard's repeated phrase: "Ignoring quantum effects" goes; the common model for a molecular bond is that of a simple harmonic oscillator. A better model is that of an anharmonic (non symetric) vibrator. This is all discussed in the "Handbook of Near-Infrared Analysis". Chapter 2. My picture of why you should add frequencies is: the "Energy" is proportional to the frequency, and it is "Energy" that we sum. | ||
| venkatarman (venkynir) Senior Member Username: venkynir Post Number: 86 Registered: 3-2004 |
Dear Horward Mark ; Thank you for very lengthy and good explanation. As an instrument engieer's with physics background( post graduate level) , I feel the absorption wave length is not fixed one.I observed it varoius few nm plus or down side . Can you explan why? .Literature and publication also report litle bit varation in absorption wavelenghts . | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 239 Registered: 9-2001 |
Venky - I was answering only the questions Iyas asked, which was specifically, the relationship between fundamentals, overtones and combination bands. I have to admit, that I made an unstated assumption, namely that we were dealing with molecular vibrations of organic materials, where it's mainly vibrations due to hydrogen that are observed. But those are, by far, the most common types of samples that are measured using NIR. There's a very nice discussion of those other issues you bring up, regarding energy levels, etc., in the first couple of chapters of "Practical Guide to Interpretive Near-Infrared Spectroscopy"; by J. Workan and L. Weyer; Taylor & Francis (CRC Press) (2008). There's no way I could reproduce that discussion here on the discussion board. As for the other question, about calculating combination and overtone frequencies, let's use an example: First of all, you have to know that in order to convert wavelength in microns (u (micrometers)) to frequency in cm-1, or vice versa, the necessary calculation is to divide 10,000 by the other unit. Thus, for example, an absorbance band with a frequency of 2,000 cm-1, converted to wavelength is 5 microns (5,000 nm). Now, to consider actual vibrations: Let's consider alipahtic -CH vibrations. The fundamental stretching vibrations are largely in the 3,000- 3,300 cm-1 region; for our example, let's consider one at 3,100 cm-1, which is 10,000/3,100 = 3.2258 u or 3225.8 nm. In order to calculate the first overtone of this, we could multiply the frequency by 2 (which gives us 2*3100 = 6,200 cm-1, ignoring QM effects) or we could divide the wavelength by 2 (3.2258/2 = 1.6129 u, or 1612.9 nm). Coverting that wavelength to frequency, we get 10,000/1.6129 = 6,200 cm-1, the same as we got by the direct calculation from the frequency. Thus we see that it doesn't matter whether we calculate the overtone frequency from the wavelength or the frequency of the fundamental. Now let's consider the calculation of a combination band from that same molecule. We know that the stretching frequency is 3100 cm-1. Let's consider a bending mode of that same atom would typically have a frequency around 1300 cm-1. The combination of these two vibrations would be at 3,100 + 1,300 = 4,400 cm-1 (again, ignoring QM effects). Converting that to wavelength give us 10000/4400 = 2.2727 u, or 2727.2 nm. Now suppose that, instead of adding frequencies, we try to add wavelengths. The wavelength for the stretching vibration is, as we saw above, 3225.8 nm. The wavlength corresponding to the bending vibration is 10000/1300 = 7.6923 u, or 7692.3 nm. If we add together those two wavelengths, we get 3225.8 + 7692.3 = 11,018.1 nm, or 11.0181 u, which when converted to frequency, is 10,000/11.0181 = 907.597 cm-1. Clearly this answer is not the same as the other one, obtained by direct addition of the frequencies. How do we know which one is correct? Study your physics and quantum mechanics, and you'll eventually come to the explanation. This is another discussion that can't be done within the confines of the space and facilities of the discussion board, so for now you'll just have to take my word for it that the answer obtained by adding frequencies is the correct answer. Howard \o/ /_\ | ||
| venkatarman (venkynir) Senior Member Username: venkynir Post Number: 85 Registered: 3-2004 |
Dear Horward Mark Good explanation . But "Another caveat to watch out for is that while overtones can be computed on the basis of either the wavelength of the frequency, combination band absorbances MUST be computed on the basis of the frequency (normally expressed, in spectroscopic usage, as wave numbers (cm-1)) and then converted back to wavelengths, if you need them in those units. " This paragraph is confusing . What about energy level , band width & penetrating power in the case of first overtone ,second and combination .Can you explain? | ||
| Don Burns (burns) Junior Member Username: burns Post Number: 7 Registered: 1-2006 |
Howard (and Iyas) - That shouldn't be confusing. The paper shows how the tall, narrow peak of plutonium in the NIR region provides a 12-fold improvement in sensitivity when compared with the standard analysis in the visible region. I still think the included diagrams would contribute to Iyes' understanding. Don Burns | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 238 Registered: 9-2001 |
Iyas - Don is talking about one of the exceptions: Lanthanides and Actinides comprise the main types of NIR absorbers that depend on low-lying electron energy levels of an inorganic type of sample, rather than the molecular vibrations that we (far) more commonly observe with NIR, in organic materials. This being the case, he's correct in saying that overtones and combinations are simply not present, since the molecular vibrations we typically refer to in that context are not present, but I think that is not responsive to your original question. Don - why are you trying to confuse the poor fellow like that???!!! \o/ /_\ | ||
| Don Burns (burns) Junior Member Username: burns Post Number: 6 Registered: 1-2006 |
Hello iyas, Way back in 1991, I addressed this subject at the FACSS meeting in Anaheim CA with a talk entitled "NIR Without Overtone/Combination Bands?" including actinides as examples. The abstract was on page 687 of the program book. If you can't find it, let me know in about a week when I'm home in NM and have my own computer; I'm now in FL. I may even have the 15-minute PPT program on a DVD. Don Burns, aka [email protected] | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 237 Registered: 9-2001 |
Iyas - while there are exceptions, the exceptions are very rare you can pretty much distinguish the differences this way: 1) Fundamentals: fundamental absorbance bands are in the mid-infrared region, that is wavelengths longer than 3� (3300 cm-1). 2) Overtones: overtones occur at integral multiples of the fundamental frequency (2x, 3x, etc). Due to quantum mechanical effects, however, the actual wavelength of an overtone band will be at a slightly longer wavelength than exactly an integral multiple. Also, the higher the overtone, the weaker the absorbance band, so that at higher overtones the bands are too weak to observe. This is the main reason why we mostly observe only those absorbance bands involving hydrogen in the NIR; for other bands, the fundamental frequency is so low (because of the high mass of the atoms involved) that only very high overtones occur in the near infrared, and they are generally too weak to measure. Because they are at such a small integral multiples, the overtones of hydrogen absorbances in the near infrared are very well separated from each other, and therefore very easy to identify. This also leaves a lot of room between them for the other (i.e., combination) bands to show up without much overlap. 3) Combination bands: combination bands are caused by the combined vibrations of the atoms involved, for example the stretching and bending vibrations of -CH2. The frequency of the absorbance band therefore equals the sum of the frequencies of the two vibrations involved, in this example the frequency of the stretching vibration plus the frequency of the bending vibration. Again, because of quantum mechanical effects it is not an exact summation. Another caveat to watch out for is that while overtones can be computed on the basis of either the wavelength of the frequency, combination band absorbances MUST be computed on the basis of the frequency (normally expressed, in spectroscopic usage, as wave numbers (cm-1)) and then converted back to wavelengths, if you need them in those units. To a large extent, the combination bands of the various hydrogen vibrations, especially for C-H vibrations, occur between the overtone bands, again making them fairly simple to identify. I hope this helps. \o/ /_\ | ||
| Michael Kinsey (mike_kinsey) Junior Member Username: mike_kinsey Post Number: 6 Registered: 1-2009 |
www.winisi.com/NIRS_theory.htm That is the website to a chart from Foss for NIr absorption, this is pretty good. Hope this helps. | ||
| iyas (iyas) Advanced Member Username: iyas Post Number: 24 Registered: 7-2007 |
dear all i am looking for an example (nir spectrum for a material ) which explains the diffrences between the fundmental ,overtone , combination bands in nir spectrum and if there some details which identify the overtone and the comabiation band and the fundemenatls on the same nir spectrum i will be very thankful regards |